diff --git a/.travis.yml b/.travis.yml
index 365ee595f..abc92c474 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,14 +7,14 @@ matrix:
   include:
     - php: 7.2
       env: WITH_PHPUNIT=true
+    - php: 7.2
+      env: WITH_PHPSTAN=true
     - php: 7.3
       env: WITH_PHPUNIT=true
     - php: 7.3
       env: WITH_PHPUNIT=true WITH_COVERAGE=true
     - php: 7.4
       env: WITH_PHPUNIT=true
-    - php: 8.0
-      env: WITH_PHPUNIT=true
 
 
 cache:
@@ -30,6 +30,9 @@ before_install:
     if [[ "$WITH_CS" != "true" ]]; then
         composer remove friendsofphp/php-cs-fixer --dev --no-update
     fi
+    if [[ "$WITH_PHPSTAN" != "true" ]]; then
+        composer remove phpstan/phpstan --dev --no-update
+    fi
   - composer validate
 
 install:
@@ -44,6 +47,10 @@ script:
     if [[ "$WITH_CS" == "true" ]]; then
         vendor/bin/php-cs-fixer fix --config=.php_cs.dist --verbose --diff --dry-run --diff-format=udiff
     fi
+  - |
+    if [[ "$WITH_PHPSTAN" == "true" ]]; then
+        vendor/bin/phpstan
+    fi
   - |
     if [[ "$WITH_PHPUNIT" == "true" ]]; then
         if [[ "$WITH_COVERAGE" == "true" ]]; then
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 5db0f6e75..b9f14400a 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -82,7 +82,28 @@ This will print a diff of proposed code style changes. To apply these suggestion
 vendor/bin/php-cs-fixer fix --config=.php_cs.dist
 ```
 
-### Step 9: Send the pull request
+You can also use
+
+```
+composer phpcs
+```
+
+### Step 9: Fix phpstan
+
+Run the following command to analyse source code.
+Make sure to fix all errors before submitting a pull request.
+
+```
+vendor/bin/phpstan analyse
+```
+
+You can also use
+
+```
+composer phpstan
+```
+
+### Step 10: Send the pull request
 
 Send the pull request from your feature branch to us. Be sure to include a description that lets us know what work you did.
 
diff --git a/composer.json b/composer.json
index 912349de9..62e4e0ef1 100644
--- a/composer.json
+++ b/composer.json
@@ -19,7 +19,8 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^8",
-        "friendsofphp/php-cs-fixer": "^2"
+        "friendsofphp/php-cs-fixer": "^2",
+        "phpstan/phpstan": "^0.12.98"
     },
     "suggest": {
         "ext-iconv": "To handle non UTF-8 CSV files (if \"php-intl\" is not already installed or is too limited)",
@@ -30,6 +31,11 @@
             "Box\\Spout\\": "src/Spout"
         }
     },
+    "scripts": {
+        "phpunit": "vendor/bin/phpunit",
+        "phpstan": "vendor/bin/phpstan analyse",
+        "phpcs": "vendor/bin/php-cs-fixer fix --config=.php_cs.dist"
+    },
     "extra": {
         "branch-alias": {
             "dev-master": "3.1.x-dev"
diff --git a/phpstan.neon b/phpstan.neon
new file mode 100644
index 000000000..7c877801d
--- /dev/null
+++ b/phpstan.neon
@@ -0,0 +1,5 @@
+parameters:
+    level: 7
+    paths:
+        - src
+        - tests
diff --git a/src/Spout/Autoloader/Psr4Autoloader.php b/src/Spout/Autoloader/Psr4Autoloader.php
index 37b6d4ac1..5c30ed784 100644
--- a/src/Spout/Autoloader/Psr4Autoloader.php
+++ b/src/Spout/Autoloader/Psr4Autoloader.php
@@ -12,7 +12,7 @@ class Psr4Autoloader
      * An associative array where the key is a namespace prefix and the value
      * is an array of base directories for classes in that namespace.
      *
-     * @var array
+     * @var array<string, array>
      */
     protected $prefixes = [];
 
diff --git a/src/Spout/Common/Entity/Cell.php b/src/Spout/Common/Entity/Cell.php
index 174831c29..660a9a0e6 100644
--- a/src/Spout/Common/Entity/Cell.php
+++ b/src/Spout/Common/Entity/Cell.php
@@ -77,7 +77,7 @@ public function __construct($value, Style $style = null)
     /**
      * @param mixed|null $value
      */
-    public function setValue($value)
+    public function setValue($value) : void
     {
         $this->value = $value;
         $this->type = $this->detectType($value);
@@ -102,7 +102,7 @@ public function getValueEvenIfError()
     /**
      * @param Style|null $style
      */
-    public function setStyle($style)
+    public function setStyle($style) : void
     {
         $this->style = $style ?: new Style();
     }
@@ -126,7 +126,7 @@ public function getType()
     /**
      * @param int $type
      */
-    public function setType($type)
+    public function setType($type) : void
     {
         $this->type = $type;
     }
diff --git a/src/Spout/Common/Entity/Row.php b/src/Spout/Common/Entity/Row.php
index 0cc49c730..bf55bed9b 100644
--- a/src/Spout/Common/Entity/Row.php
+++ b/src/Spout/Common/Entity/Row.php
@@ -95,7 +95,10 @@ public function getNumCells()
             return 0;
         }
 
-        return \max(\array_keys($this->cells)) + 1;
+        /** @var int $highest */
+        $highest = \max(\array_keys($this->cells));
+
+        return $highest + 1;
     }
 
     /**
@@ -118,7 +121,7 @@ public function setStyle($style)
     }
 
     /**
-     * @return array The row values, as array
+     * @return array<mixed> The row values, as array
      */
     public function toArray()
     {
diff --git a/src/Spout/Common/Entity/Style/Border.php b/src/Spout/Common/Entity/Style/Border.php
index bbf43ad30..3ab6e208a 100644
--- a/src/Spout/Common/Entity/Style/Border.php
+++ b/src/Spout/Common/Entity/Style/Border.php
@@ -22,11 +22,11 @@ class Border
     const WIDTH_MEDIUM = 'medium';
     const WIDTH_THICK = 'thick';
 
-    /** @var array A list of BorderPart objects for this border. */
+    /** @var array<BorderPart> A list of BorderPart objects for this border. */
     private $parts = [];
 
     /**
-     * @param array $borderParts
+     * @param array<BorderPart> $borderParts
      */
     public function __construct(array $borderParts = [])
     {
@@ -52,7 +52,7 @@ public function hasPart($name)
     }
 
     /**
-     * @return array
+     * @return array<BorderPart>
      */
     public function getParts()
     {
@@ -61,7 +61,7 @@ public function getParts()
 
     /**
      * Set BorderParts
-     * @param array $parts
+     * @param array<BorderPart> $parts
      * @return void
      */
     public function setParts($parts)
diff --git a/src/Spout/Common/Entity/Style/BorderPart.php b/src/Spout/Common/Entity/Style/BorderPart.php
index c0874847e..be433fc03 100644
--- a/src/Spout/Common/Entity/Style/BorderPart.php
+++ b/src/Spout/Common/Entity/Style/BorderPart.php
@@ -32,7 +32,7 @@ class BorderPart
     protected $width;
 
     /**
-     * @var array Allowed style constants for parts.
+     * @var array<string> Allowed style constants for parts.
      */
     protected static $allowedStyles = [
         'none',
@@ -43,7 +43,7 @@ class BorderPart
     ];
 
     /**
-     * @var array Allowed names constants for border parts.
+     * @var array<string> Allowed names constants for border parts.
      */
     protected static $allowedNames = [
         'left',
@@ -53,7 +53,7 @@ class BorderPart
     ];
 
     /**
-     * @var array Allowed width constants for border parts.
+     * @var array<string> Allowed width constants for border parts.
      */
     protected static $allowedWidths = [
         'thin',
@@ -159,7 +159,7 @@ public function setWidth($width)
     }
 
     /**
-     * @return array
+     * @return array<string>
      */
     public static function getAllowedStyles()
     {
@@ -167,7 +167,7 @@ public static function getAllowedStyles()
     }
 
     /**
-     * @return array
+     * @return array<string>
      */
     public static function getAllowedNames()
     {
@@ -175,7 +175,7 @@ public static function getAllowedNames()
     }
 
     /**
-     * @return array
+     * @return array<string>
      */
     public static function getAllowedWidths()
     {
diff --git a/src/Spout/Common/Entity/Style/CellAlignment.php b/src/Spout/Common/Entity/Style/CellAlignment.php
index 60fe83309..ed47f8470 100644
--- a/src/Spout/Common/Entity/Style/CellAlignment.php
+++ b/src/Spout/Common/Entity/Style/CellAlignment.php
@@ -13,6 +13,9 @@ abstract class CellAlignment
     const CENTER = 'center';
     const JUSTIFY = 'justify';
 
+    /**
+     * @var array<string, int>
+     */
     private static $VALID_ALIGNMENTS = [
         self::LEFT => 1,
         self::RIGHT => 1,
diff --git a/src/Spout/Common/Entity/Style/Style.php b/src/Spout/Common/Entity/Style/Style.php
index 2bacc7afa..3fc020dea 100644
--- a/src/Spout/Common/Entity/Style/Style.php
+++ b/src/Spout/Common/Entity/Style/Style.php
@@ -66,7 +66,7 @@ class Style
     /** @var bool Whether the wrap text property was set */
     private $hasSetWrapText = false;
 
-    /** @var Border */
+    /** @var Border|null */
     private $border;
 
     /** @var bool Whether border properties should be applied */
@@ -110,7 +110,7 @@ public function setId($id)
     }
 
     /**
-     * @return Border
+     * @return Border|null
      */
     public function getBorder()
     {
diff --git a/src/Spout/Common/Helper/CellTypeHelper.php b/src/Spout/Common/Helper/CellTypeHelper.php
index 97d2e8dda..74e18f330 100644
--- a/src/Spout/Common/Helper/CellTypeHelper.php
+++ b/src/Spout/Common/Helper/CellTypeHelper.php
@@ -9,7 +9,7 @@
 class CellTypeHelper
 {
     /**
-     * @param $value
+     * @param mixed $value
      * @return bool Whether the given value is considered "empty"
      */
     public static function isEmpty($value)
@@ -18,7 +18,7 @@ public static function isEmpty($value)
     }
 
     /**
-     * @param $value
+     * @param mixed $value
      * @return bool Whether the given value is a non empty string
      */
     public static function isNonEmptyString($value)
@@ -30,7 +30,7 @@ public static function isNonEmptyString($value)
      * Returns whether the given value is numeric.
      * A numeric value is from type "integer" or "double" ("float" is not returned by gettype).
      *
-     * @param $value
+     * @param mixed $value
      * @return bool Whether the given value is numeric
      */
     public static function isNumeric($value)
@@ -44,7 +44,7 @@ public static function isNumeric($value)
      * Returns whether the given value is boolean.
      * "true"/"false" and 0/1 are not booleans.
      *
-     * @param $value
+     * @param mixed $value
      * @return bool Whether the given value is boolean
      */
     public static function isBoolean($value)
@@ -55,7 +55,7 @@ public static function isBoolean($value)
     /**
      * Returns whether the given value is a DateTime or DateInterval object.
      *
-     * @param $value
+     * @param mixed $value
      * @return bool Whether the given value is a DateTime or DateInterval object
      */
     public static function isDateTimeOrDateInterval($value)
diff --git a/src/Spout/Common/Helper/EncodingHelper.php b/src/Spout/Common/Helper/EncodingHelper.php
index 62ddf60bd..8ace6d3f2 100644
--- a/src/Spout/Common/Helper/EncodingHelper.php
+++ b/src/Spout/Common/Helper/EncodingHelper.php
@@ -27,7 +27,7 @@ class EncodingHelper
     /** @var \Box\Spout\Common\Helper\GlobalFunctionsHelper Helper to work with global functions */
     protected $globalFunctionsHelper;
 
-    /** @var array Map representing the encodings supporting BOMs (key) and their associated BOM (value) */
+    /** @var array<string, string> Map representing the encodings supporting BOMs (key) and their associated BOM (value) */
     protected $supportedEncodingsWithBom;
 
     /**
@@ -143,7 +143,7 @@ protected function attemptConversion($string, $sourceEncoding, $targetEncoding)
             throw new EncodingConversionException("The conversion from $sourceEncoding to $targetEncoding is not supported. Please install \"iconv\" or \"PHP Intl\".");
         }
 
-        if ($convertedString === false) {
+        if (!is_string($convertedString)) {
             throw new EncodingConversionException("The conversion from $sourceEncoding to $targetEncoding failed.");
         }
 
diff --git a/src/Spout/Common/Helper/Escaper/XLSX.php b/src/Spout/Common/Helper/Escaper/XLSX.php
index daef46e4d..6fef6c778 100644
--- a/src/Spout/Common/Helper/Escaper/XLSX.php
+++ b/src/Spout/Common/Helper/Escaper/XLSX.php
@@ -22,6 +22,7 @@ class XLSX implements EscaperInterface
 
     /**
      * Initializes the control characters if not already done
+     * @return void
      */
     protected function initIfNeeded()
     {
diff --git a/src/Spout/Common/Helper/FileSystemHelper.php b/src/Spout/Common/Helper/FileSystemHelper.php
index 4d21fd38c..fed9bb838 100644
--- a/src/Spout/Common/Helper/FileSystemHelper.php
+++ b/src/Spout/Common/Helper/FileSystemHelper.php
@@ -19,7 +19,10 @@ class FileSystemHelper implements FileSystemHelperInterface
      */
     public function __construct(string $baseFolderPath)
     {
-        $this->baseFolderRealPath = \realpath($baseFolderPath);
+        /** @var string $realPath */
+        $realPath = \realpath($baseFolderPath);
+
+        $this->baseFolderRealPath = $realPath;
     }
 
     /**
@@ -127,6 +130,7 @@ protected function throwIfOperationNotInBaseFolder(string $operationFolderPath)
         if (!$this->baseFolderRealPath) {
             throw new IOException("The base folder path is invalid: {$this->baseFolderRealPath}");
         }
+        /** @var string $operationFolderRealPath */
         $isInBaseFolder = (\strpos($operationFolderRealPath, $this->baseFolderRealPath) === 0);
         if (!$isInBaseFolder) {
             throw new IOException("Cannot perform I/O operation outside of the base folder: {$this->baseFolderRealPath}");
diff --git a/src/Spout/Common/Helper/GlobalFunctionsHelper.php b/src/Spout/Common/Helper/GlobalFunctionsHelper.php
index 5deb8d93f..ea4300019 100644
--- a/src/Spout/Common/Helper/GlobalFunctionsHelper.php
+++ b/src/Spout/Common/Helper/GlobalFunctionsHelper.php
@@ -33,7 +33,10 @@ public function fopen($fileName, $mode)
      */
     public function fgets($handle, $length = null)
     {
-        return \fgets($handle, $length);
+        /** @var string $fgets */
+        $fgets =  \fgets($handle, $length);
+
+        return $fgets;
     }
 
     /**
@@ -46,7 +49,10 @@ public function fgets($handle, $length = null)
      */
     public function fputs($handle, $string)
     {
-        return \fputs($handle, $string);
+        /** @var int $fputs */
+        $fputs = \fputs($handle, $string);
+
+        return $fputs;
     }
 
     /**
@@ -82,7 +88,7 @@ public function fseek($handle, $offset)
      * @param int|null $length
      * @param string|null $delimiter
      * @param string|null $enclosure
-     * @return array
+     * @return array<mixed>|false
      */
     public function fgetcsv($handle, $length = null, $delimiter = null, $enclosure = null)
     {
@@ -100,10 +106,10 @@ public function fgetcsv($handle, $length = null, $delimiter = null, $enclosure =
      * @see fputcsv()
      *
      * @param resource $handle
-     * @param array $fields
+     * @param array<mixed> $fields
      * @param string|null $delimiter
      * @param string|null $enclosure
-     * @return int
+     * @return int|false
      */
     public function fputcsv($handle, array $fields, $delimiter = null, $enclosure = null)
     {
@@ -126,7 +132,10 @@ public function fputcsv($handle, array $fields, $delimiter = null, $enclosure =
      */
     public function fwrite($handle, $string)
     {
-        return \fwrite($handle, $string);
+        /** @var int $fwrite */
+        $fwrite = \fwrite($handle, $string);
+
+        return $fwrite;
     }
 
     /**
@@ -176,7 +185,10 @@ public function file_get_contents($filePath)
     {
         $realFilePath = $this->convertToUseRealPath($filePath);
 
-        return \file_get_contents($realFilePath);
+        /** @var string $content */
+        $content = \file_get_contents($realFilePath);
+
+        return $content;
     }
 
     /**
@@ -197,7 +209,9 @@ protected function convertToUseRealPath($filePath)
                 $realFilePath = 'zip://' . \realpath($documentPath) . '#' . $documentInsideZipPath;
             }
         } else {
-            $realFilePath = \realpath($filePath);
+            /** @var string $realPath */
+            $realPath = \realpath($filePath);
+            $realFilePath = $realPath;
         }
 
         return $realFilePath;
@@ -308,7 +322,7 @@ public function mb_convert_encoding($string, $sourceEncoding, $targetEncoding)
      * Wrapper around global function stream_get_wrappers()
      * @see stream_get_wrappers()
      *
-     * @return array
+     * @return array<int, string>
      */
     public function stream_get_wrappers()
     {
diff --git a/src/Spout/Common/Helper/StringHelper.php b/src/Spout/Common/Helper/StringHelper.php
index 6256b1e5f..84996df37 100644
--- a/src/Spout/Common/Helper/StringHelper.php
+++ b/src/Spout/Common/Helper/StringHelper.php
@@ -16,7 +16,7 @@ class StringHelper
     /** @var bool Whether the code is running with PHP7 or older versions */
     private $isRunningPhp7OrOlder;
 
-    /** @var array Locale info, used for number formatting */
+    /** @var array<string, mixed> Locale info, used for number formatting */
     private $localeInfo;
 
     /**
@@ -93,13 +93,16 @@ public function getCharLastOccurrencePosition($char, $string)
     public function formatNumericValue($numericValue)
     {
         if ($this->isRunningPhp7OrOlder && is_float($numericValue)) {
-            return str_replace(
+            /** @var string $replaced */
+            $replaced =  str_replace(
                 [$this->localeInfo['thousands_sep'], $this->localeInfo['decimal_point']],
                 ['', '.'],
-                $numericValue
+                (string) $numericValue
             );
+
+            return $replaced;
         }
 
-        return $numericValue;
+        return (string) $numericValue;
     }
 }
diff --git a/src/Spout/Common/Manager/OptionsManagerAbstract.php b/src/Spout/Common/Manager/OptionsManagerAbstract.php
index 5670b95be..d8a853eb6 100644
--- a/src/Spout/Common/Manager/OptionsManagerAbstract.php
+++ b/src/Spout/Common/Manager/OptionsManagerAbstract.php
@@ -12,7 +12,7 @@ abstract class OptionsManagerAbstract implements OptionsManagerInterface
     /** @var string[] List of all supported option names */
     private $supportedOptions = [];
 
-    /** @var array Associative array [OPTION_NAME => OPTION_VALUE] */
+    /** @var array<string, mixed> Associative array [OPTION_NAME => OPTION_VALUE] */
     private $options = [];
 
     /**
@@ -25,7 +25,7 @@ public function __construct()
     }
 
     /**
-     * @return array List of supported options
+     * @return array<string> List of supported options
      */
     abstract protected function getSupportedOptions();
 
diff --git a/src/Spout/Reader/CSV/Creator/InternalEntityFactory.php b/src/Spout/Reader/CSV/Creator/InternalEntityFactory.php
index 65cf3eb7e..57291da47 100644
--- a/src/Spout/Reader/CSV/Creator/InternalEntityFactory.php
+++ b/src/Spout/Reader/CSV/Creator/InternalEntityFactory.php
@@ -84,7 +84,7 @@ public function createCell($cellValue)
     }
 
     /**
-     * @param array $cellValues
+     * @param array<mixed> $cellValues
      * @return Row
      */
     public function createRowFromArray(array $cellValues = [])
diff --git a/src/Spout/Reader/CSV/Reader.php b/src/Spout/Reader/CSV/Reader.php
index ca61ef2e2..1dc0b4f38 100644
--- a/src/Spout/Reader/CSV/Reader.php
+++ b/src/Spout/Reader/CSV/Reader.php
@@ -13,7 +13,7 @@
  */
 class Reader extends ReaderAbstract
 {
-    /** @var resource Pointer to the file to be written */
+    /** @var resource|null Pointer to the file to be written */
     protected $filePointer;
 
     /** @var SheetIterator To iterator over the CSV unique "sheet" */
@@ -84,13 +84,16 @@ protected function doesSupportStreamWrapper()
      */
     protected function openReader($filePath)
     {
-        $this->originalAutoDetectLineEndings = \ini_get('auto_detect_line_endings');
+        /** @var string $autoDetectLineEndings */
+        $autoDetectLineEndings = \ini_get('auto_detect_line_endings');
+        $this->originalAutoDetectLineEndings = $autoDetectLineEndings;
         \ini_set('auto_detect_line_endings', '1');
 
-        $this->filePointer = $this->globalFunctionsHelper->fopen($filePath, 'r');
-        if (!$this->filePointer) {
+        $filePointer = $this->globalFunctionsHelper->fopen($filePath, 'r');
+        if (!is_resource($filePointer)) {
             throw new IOException("Could not open file $filePath for reading.");
         }
+        $this->filePointer = $filePointer;
 
         /** @var InternalEntityFactory $entityFactory */
         $entityFactory = $this->entityFactory;
diff --git a/src/Spout/Reader/CSV/RowIterator.php b/src/Spout/Reader/CSV/RowIterator.php
index 78e665013..c022e6458 100644
--- a/src/Spout/Reader/CSV/RowIterator.php
+++ b/src/Spout/Reader/CSV/RowIterator.php
@@ -13,6 +13,7 @@
 /**
  * Class RowIterator
  * Iterate over CSV rows.
+ * @implements IteratorInterface<Row>
  */
 class RowIterator implements IteratorInterface
 {
@@ -21,7 +22,7 @@ class RowIterator implements IteratorInterface
      */
     const MAX_READ_BYTES_PER_LINE = 0;
 
-    /** @var resource Pointer to the CSV file to read */
+    /** @var resource|null Pointer to the CSV file to read */
     protected $filePointer;
 
     /** @var int Number of read rows */
@@ -147,6 +148,7 @@ protected function readDataForNextRow()
 
         if ($rowData !== false) {
             // str_replace will replace NULL values by empty strings
+            /** @phpstan-ignore-next-line */
             $rowDataBufferAsArray = \str_replace(null, null, $rowData);
             $this->rowBuffer = $this->entityFactory->createRowFromArray($rowDataBufferAsArray);
             $this->numReadRows++;
@@ -158,7 +160,7 @@ protected function readDataForNextRow()
     }
 
     /**
-     * @param array|bool $currentRowData
+     * @param array<int, string>|bool $currentRowData
      * @return bool Whether the data for the current row can be returned or if we need to keep reading
      */
     protected function shouldReadNextRow($currentRowData)
@@ -179,7 +181,7 @@ protected function shouldReadNextRow($currentRowData)
      * we remove manually whitespace with ltrim or rtrim (depending on the order of the bytes)
      *
      * @throws \Box\Spout\Common\Exception\EncodingConversionException If unable to convert data to UTF-8
-     * @return array|false The row for the current file pointer, encoded in UTF-8 or FALSE if nothing to read
+     * @return array<int, string>|false The row for the current file pointer, encoded in UTF-8 or FALSE if nothing to read
      */
     protected function getNextUTF8EncodedRow()
     {
@@ -210,7 +212,7 @@ protected function getNextUTF8EncodedRow()
     }
 
     /**
-     * @param array|bool $lineData Array containing the cells value for the line
+     * @param array<mixed>|bool $lineData Array containing the cells value for the line
      * @return bool Whether the given line is empty
      */
     protected function isEmptyLine($lineData)
diff --git a/src/Spout/Reader/CSV/SheetIterator.php b/src/Spout/Reader/CSV/SheetIterator.php
index 69eb58a0b..dcf96d342 100644
--- a/src/Spout/Reader/CSV/SheetIterator.php
+++ b/src/Spout/Reader/CSV/SheetIterator.php
@@ -7,6 +7,7 @@
 /**
  * Class SheetIterator
  * Iterate over CSV unique "sheet".
+ * @implements IteratorInterface<Sheet>
  */
 class SheetIterator implements IteratorInterface
 {
diff --git a/src/Spout/Reader/Common/Creator/ReaderEntityFactory.php b/src/Spout/Reader/Common/Creator/ReaderEntityFactory.php
index 06a9d9fbe..2c5d74ada 100644
--- a/src/Spout/Reader/Common/Creator/ReaderEntityFactory.php
+++ b/src/Spout/Reader/Common/Creator/ReaderEntityFactory.php
@@ -2,7 +2,6 @@
 
 namespace Box\Spout\Reader\Common\Creator;
 
-use Box\Spout\Common\Exception\UnsupportedTypeException;
 use Box\Spout\Common\Type;
 use Box\Spout\Reader\ReaderInterface;
 
@@ -31,11 +30,10 @@ public static function createReaderFromFile(string $path)
      */
     public static function createCSVReader()
     {
-        try {
-            return ReaderFactory::createFromType(Type::CSV);
-        } catch (UnsupportedTypeException $e) {
-            // should never happen
-        }
+        /** @var \Box\Spout\Reader\CSV\Reader $csvReader */
+        $csvReader = ReaderFactory::createFromType(Type::CSV);
+
+        return $csvReader;
     }
 
     /**
@@ -45,11 +43,10 @@ public static function createCSVReader()
      */
     public static function createXLSXReader()
     {
-        try {
-            return ReaderFactory::createFromType(Type::XLSX);
-        } catch (UnsupportedTypeException $e) {
-            // should never happen
-        }
+        /** @var \Box\Spout\Reader\XLSX\Reader $xlsxReader */
+        $xlsxReader =  ReaderFactory::createFromType(Type::XLSX);
+
+        return $xlsxReader;
     }
 
     /**
@@ -59,10 +56,9 @@ public static function createXLSXReader()
      */
     public static function createODSReader()
     {
-        try {
-            return ReaderFactory::createFromType(Type::ODS);
-        } catch (UnsupportedTypeException $e) {
-            // should never happen
-        }
+        /** @var \Box\Spout\Reader\ODS\Reader $odsReader */
+        $odsReader = ReaderFactory::createFromType(Type::ODS);
+
+        return $odsReader;
     }
 }
diff --git a/src/Spout/Reader/Common/XMLProcessor.php b/src/Spout/Reader/Common/XMLProcessor.php
index 05ee970b5..2ac3f96a6 100644
--- a/src/Spout/Reader/Common/XMLProcessor.php
+++ b/src/Spout/Reader/Common/XMLProcessor.php
@@ -25,7 +25,7 @@ class XMLProcessor
     /** @var \Box\Spout\Reader\Wrapper\XMLReader The XMLReader object that will help read sheet's XML data */
     protected $xmlReader;
 
-    /** @var array Registered callbacks */
+    /** @var array<string, array> Registered callbacks */
     private $callbacks = [];
 
     /**
@@ -39,7 +39,7 @@ public function __construct($xmlReader)
     /**
      * @param string $nodeName A callback may be triggered when a node with this name is read
      * @param int $nodeType Type of the node [NODE_TYPE_START || NODE_TYPE_END]
-     * @param callable $callback Callback to execute when the read node has the given name and type
+     * @param array{object, string} $callback Callback to execute when the read node has the given name and type
      * @return XMLProcessor
      */
     public function registerCallback($nodeName, $nodeType, $callback)
@@ -66,8 +66,8 @@ private function getCallbackKey($nodeName, $nodeType)
      * Since some functions can be called a lot, we pre-process the callback to only return the elements that
      * will be needed to invoke the callback later.
      *
-     * @param callable $callback Array reference to a callback: [OBJECT, METHOD_NAME]
-     * @return array Associative array containing the elements needed to invoke the callback using Reflection
+     * @param array{object, string} $callback Array reference to a callback: [OBJECT, METHOD_NAME]
+     * @return array<string, mixed> Associative array containing the elements needed to invoke the callback using Reflection
      */
     private function getInvokableCallbackData($callback)
     {
@@ -113,7 +113,7 @@ public function readUntilStopped()
      * @param string $nodeNamePossiblyWithPrefix Name of the node, possibly prefixed
      * @param string $nodeNameWithoutPrefix Name of the same node, un-prefixed
      * @param int $nodeType Type of the node [NODE_TYPE_START || NODE_TYPE_END]
-     * @return array|null Callback data to be used for execution when a node of the given name/type is read or NULL if none found
+     * @return array<mixed>|null Callback data to be used for execution when a node of the given name/type is read or NULL if none found
      */
     private function getRegisteredCallbackData($nodeNamePossiblyWithPrefix, $nodeNameWithoutPrefix, $nodeType)
     {
@@ -134,8 +134,8 @@ private function getRegisteredCallbackData($nodeNamePossiblyWithPrefix, $nodeNam
     }
 
     /**
-     * @param array $callbackData Associative array containing data to invoke the callback using Reflection
-     * @param array $args Arguments to pass to the callback
+     * @param array<string, mixed> $callbackData Associative array containing data to invoke the callback using Reflection
+     * @param array<mixed> $args Arguments to pass to the callback
      * @return int Callback response
      */
     private function invokeCallback($callbackData, $args)
diff --git a/src/Spout/Reader/IteratorInterface.php b/src/Spout/Reader/IteratorInterface.php
index 4a9305d98..48ce233dd 100644
--- a/src/Spout/Reader/IteratorInterface.php
+++ b/src/Spout/Reader/IteratorInterface.php
@@ -4,6 +4,8 @@
 
 /**
  * Interface IteratorInterface
+ * @template TValue
+ * @extends  \Iterator<int, TValue>
  */
 interface IteratorInterface extends \Iterator
 {
diff --git a/src/Spout/Reader/ODS/Creator/InternalEntityFactory.php b/src/Spout/Reader/ODS/Creator/InternalEntityFactory.php
index cb06ce207..a9d96d6ee 100644
--- a/src/Spout/Reader/ODS/Creator/InternalEntityFactory.php
+++ b/src/Spout/Reader/ODS/Creator/InternalEntityFactory.php
@@ -105,7 +105,7 @@ public function createXMLReader()
     }
 
     /**
-     * @param $xmlReader
+     * @param XMLReader $xmlReader
      * @return XMLProcessor
      */
     private function createXMLProcessor($xmlReader)
diff --git a/src/Spout/Reader/ODS/Helper/CellValueFormatter.php b/src/Spout/Reader/ODS/Helper/CellValueFormatter.php
index 168770f10..ba2edc9b1 100644
--- a/src/Spout/Reader/ODS/Helper/CellValueFormatter.php
+++ b/src/Spout/Reader/ODS/Helper/CellValueFormatter.php
@@ -43,7 +43,7 @@ class CellValueFormatter
     /** @var \Box\Spout\Common\Helper\Escaper\ODS Used to unescape XML data */
     protected $escaper;
 
-    /** @var array List of XML nodes representing whitespaces and their corresponding value */
+    /** @var array<string, string> List of XML nodes representing whitespaces and their corresponding value */
     private static $WHITESPACE_XML_NODES = [
         self::XML_NODE_TEXT_S => ' ',
         self::XML_NODE_TEXT_TAB => "\t",
@@ -64,7 +64,7 @@ public function __construct($shouldFormatDates, $escaper)
      * Returns the (unescaped) correctly marshalled, cell value associated to the given XML node.
      * @see http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#refTable13
      *
-     * @param \DOMNode $node
+     * @param \DOMElement $node
      * @throws InvalidValueException If the node value is not valid
      * @return string|int|float|bool|\DateTime|\DateInterval The value associated with the cell, empty string if cell's type is void/undefined
      */
@@ -96,7 +96,7 @@ public function extractAndFormatNodeValue($node)
     /**
      * Returns the cell String value.
      *
-     * @param \DOMNode $node
+     * @param \DOMElement $node
      * @return string The value associated with the cell
      */
     protected function formatStringCellValue($node)
@@ -115,7 +115,7 @@ protected function formatStringCellValue($node)
     }
 
     /**
-     * @param $pNode
+     * @param \DOMNode $pNode
      * @return string
      */
     private function extractTextValueFromNode($pNode)
@@ -159,7 +159,7 @@ private function isWhitespaceNode($nodeName)
      *
      * @see https://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#__RefHeading__1415200_253892949
      *
-     * @param \DOMNode $node The XML node representing a whitespace
+     * @param \DOMElement $node The XML node representing a whitespace
      * @return string The corresponding whitespace value
      */
     private function transformWhitespaceNode($node)
@@ -173,7 +173,7 @@ private function transformWhitespaceNode($node)
     /**
      * Returns the cell Numeric value from the given node.
      *
-     * @param \DOMNode $node
+     * @param \DOMElement $node
      * @return int|float The value associated with the cell
      */
     protected function formatFloatCellValue($node)
@@ -190,7 +190,7 @@ protected function formatFloatCellValue($node)
     /**
      * Returns the cell Boolean value from the given node.
      *
-     * @param \DOMNode $node
+     * @param \DOMElement $node
      * @return bool The value associated with the cell
      */
     protected function formatBooleanCellValue($node)
@@ -203,7 +203,7 @@ protected function formatBooleanCellValue($node)
     /**
      * Returns the cell Date value from the given node.
      *
-     * @param \DOMNode $node
+     * @param \DOMElement $node
      * @throws InvalidValueException If the value is not a valid date
      * @return \DateTime|string The value associated with the cell
      */
@@ -234,7 +234,7 @@ protected function formatDateCellValue($node)
     /**
      * Returns the cell Time value from the given node.
      *
-     * @param \DOMNode $node
+     * @param \DOMElement $node
      * @throws InvalidValueException If the value is not a valid time
      * @return \DateInterval|string The value associated with the cell
      */
@@ -265,7 +265,7 @@ protected function formatTimeCellValue($node)
     /**
      * Returns the cell Currency value from the given node.
      *
-     * @param \DOMNode $node
+     * @param \DOMElement $node
      * @return string The value associated with the cell (e.g. "100 USD" or "9.99 EUR")
      */
     protected function formatCurrencyCellValue($node)
@@ -279,7 +279,7 @@ protected function formatCurrencyCellValue($node)
     /**
      * Returns the cell Percentage value from the given node.
      *
-     * @param \DOMNode $node
+     * @param \DOMElement $node
      * @return int|float The value associated with the cell
      */
     protected function formatPercentageCellValue($node)
diff --git a/src/Spout/Reader/ODS/Reader.php b/src/Spout/Reader/ODS/Reader.php
index d07dbb43c..9e3f4d1d2 100644
--- a/src/Spout/Reader/ODS/Reader.php
+++ b/src/Spout/Reader/ODS/Reader.php
@@ -12,7 +12,7 @@
  */
 class Reader extends ReaderAbstract
 {
-    /** @var \ZipArchive */
+    /** @var \ZipArchive|null */
     protected $zip;
 
     /** @var SheetIterator To iterator over the ODS sheets */
diff --git a/src/Spout/Reader/ODS/RowIterator.php b/src/Spout/Reader/ODS/RowIterator.php
index 81f0953d8..a0c348f85 100644
--- a/src/Spout/Reader/ODS/RowIterator.php
+++ b/src/Spout/Reader/ODS/RowIterator.php
@@ -19,6 +19,7 @@
 
 /**
  * Class RowIterator
+ * @implements IteratorInterface<Row>
  */
 class RowIterator implements IteratorInterface
 {
@@ -56,7 +57,7 @@ class RowIterator implements IteratorInterface
     /** @var Row The currently processed row */
     protected $currentlyProcessedRow;
 
-    /** @var Row Buffer used to store the current row, while checking if there are more rows to read */
+    /** @var Row|null Buffer used to store the current row, while checking if there are more rows to read */
     protected $rowBuffer;
 
     /** @var bool Indicates whether all rows have been read */
@@ -68,7 +69,7 @@ class RowIterator implements IteratorInterface
     /** @var int Row index to be processed next (one-based) */
     protected $nextRowIndexToBeProcessed = 1;
 
-    /** @var Cell Last processed cell (because when reading cell at column N+1, cell N is processed) */
+    /** @var Cell|null Last processed cell (because when reading cell at column N+1, cell N is processed) */
     protected $lastProcessedCell;
 
     /** @var int Number of times the last processed row should be repeated */
@@ -225,6 +226,7 @@ protected function processCellStartingNode($xmlReader)
         $currentNumColumnsRepeated = $this->getNumColumnsRepeatedForCurrentNode($xmlReader);
 
         // NOTE: expand() will automatically decode all XML entities of the child nodes
+        /** @var \DOMElement $node */
         $node = $xmlReader->expand();
         $currentCell = $this->getCell($node);
 
@@ -316,7 +318,7 @@ protected function getNumColumnsRepeatedForCurrentNode($xmlReader)
     /**
      * Returns the cell with (unescaped) correctly marshalled, cell value associated to the given XML node.
      *
-     * @param \DOMNode $node
+     * @param \DOMElement $node
      * @return Cell The cell set with the associated with the cell
      */
     protected function getCell($node)
@@ -339,7 +341,7 @@ protected function getCell($node)
      * row data yet (as we still need to apply the "num-columns-repeated" attribute).
      *
      * @param Row $currentRow
-     * @param Cell $lastReadCell The last read cell
+     * @param Cell|null $lastReadCell The last read cell
      * @return bool Whether the row is empty
      */
     protected function isEmptyRow($currentRow, $lastReadCell)
diff --git a/src/Spout/Reader/ODS/SheetIterator.php b/src/Spout/Reader/ODS/SheetIterator.php
index c7b8cd9dc..7f9a45344 100644
--- a/src/Spout/Reader/ODS/SheetIterator.php
+++ b/src/Spout/Reader/ODS/SheetIterator.php
@@ -12,6 +12,7 @@
 /**
  * Class SheetIterator
  * Iterate over ODS sheet.
+ * @implements IteratorInterface<Sheet>
  */
 class SheetIterator implements IteratorInterface
 {
@@ -52,7 +53,7 @@ class SheetIterator implements IteratorInterface
     /** @var string The name of the sheet that was defined as active */
     protected $activeSheetName;
 
-    /** @var array Associative array [STYLE_NAME] => [IS_SHEET_VISIBLE] */
+    /** @var array<string, bool> Associative array [STYLE_NAME] => [IS_SHEET_VISIBLE] */
     protected $sheetsVisibility;
 
     /**
@@ -101,13 +102,14 @@ public function rewind()
     /**
      * Extracts the visibility of the sheets
      *
-     * @return array Associative array [STYLE_NAME] => [IS_SHEET_VISIBLE]
+     * @return array<string, bool> Associative array [STYLE_NAME] => [IS_SHEET_VISIBLE]
      */
     private function readSheetsVisibility()
     {
         $sheetsVisibility = [];
 
         $this->xmlReader->readUntilNodeFound(self::XML_NODE_AUTOMATIC_STYLES);
+        /** @var \DOMElement $automaticStylesNode */
         $automaticStylesNode = $this->xmlReader->expand();
 
         $tableStyleNodes = $automaticStylesNode->getElementsByTagNameNS(self::XML_STYLE_NAMESPACE, self::XML_NODE_STYLE_TABLE_PROPERTIES);
diff --git a/src/Spout/Reader/ReaderAbstract.php b/src/Spout/Reader/ReaderAbstract.php
index 39b333d4c..4156f457b 100644
--- a/src/Spout/Reader/ReaderAbstract.php
+++ b/src/Spout/Reader/ReaderAbstract.php
@@ -45,15 +45,15 @@ abstract protected function openReader($filePath);
 
     /**
      * Returns an iterator to iterate over sheets.
-     *
-     * @return IteratorInterface To iterate over sheets
+     * @template T
+     * @return IteratorInterface<T>|null To iterate over sheets
      */
     abstract protected function getConcreteSheetIterator();
 
     /**
      * Closes the reader. To be used after reading the file.
      *
-     * @return ReaderAbstract
+     * @return void
      */
     abstract protected function closeReader();
 
@@ -145,7 +145,10 @@ protected function getFileRealPath($filePath)
         }
 
         // Need to use realpath to fix "Can't open file" on some Windows setup
-        return \realpath($filePath);
+        /** @var string $path */
+        $path = \realpath($filePath);
+
+        return $path;
     }
 
     /**
@@ -211,7 +214,8 @@ protected function isPhpStream($filePath)
      * Returns an iterator to iterate over sheets.
      *
      * @throws \Box\Spout\Reader\Exception\ReaderNotOpenedException If called before opening the reader
-     * @return \Iterator To iterate over sheets
+     * @template T
+     * @return IteratorInterface<T> To iterate over sheets
      */
     public function getSheetIterator()
     {
diff --git a/src/Spout/Reader/ReaderInterface.php b/src/Spout/Reader/ReaderInterface.php
index 74bcc4a99..bbd9f4aee 100644
--- a/src/Spout/Reader/ReaderInterface.php
+++ b/src/Spout/Reader/ReaderInterface.php
@@ -21,7 +21,7 @@ public function open($filePath);
      * Returns an iterator to iterate over sheets.
      *
      * @throws \Box\Spout\Reader\Exception\ReaderNotOpenedException If called before opening the reader
-     * @return \Iterator To iterate over sheets
+     * @return \Iterator<SheetInterface> To iterate over sheets
      */
     public function getSheetIterator();
 
diff --git a/src/Spout/Reader/SheetInterface.php b/src/Spout/Reader/SheetInterface.php
index f77ecdf5e..99a1c8563 100644
--- a/src/Spout/Reader/SheetInterface.php
+++ b/src/Spout/Reader/SheetInterface.php
@@ -2,13 +2,15 @@
 
 namespace Box\Spout\Reader;
 
+use Box\Spout\Common\Entity\Row;
+
 /**
  * Interface SheetInterface
  */
 interface SheetInterface
 {
     /**
-     * @return IteratorInterface Iterator to iterate over the sheet's rows.
+     * @return IteratorInterface<Row> Iterator to iterate over the sheet's rows.
      */
     public function getRowIterator();
 
diff --git a/src/Spout/Reader/XLSX/Creator/InternalEntityFactory.php b/src/Spout/Reader/XLSX/Creator/InternalEntityFactory.php
index 4dc2dc7d9..d5ab4c98c 100644
--- a/src/Spout/Reader/XLSX/Creator/InternalEntityFactory.php
+++ b/src/Spout/Reader/XLSX/Creator/InternalEntityFactory.php
@@ -152,7 +152,7 @@ public function createXMLReader()
     }
 
     /**
-     * @param $xmlReader
+     * @param XMLReader $xmlReader
      * @return XMLProcessor
      */
     public function createXMLProcessor($xmlReader)
diff --git a/src/Spout/Reader/XLSX/Helper/CellHelper.php b/src/Spout/Reader/XLSX/Helper/CellHelper.php
index d970189e6..01c3faf73 100644
--- a/src/Spout/Reader/XLSX/Helper/CellHelper.php
+++ b/src/Spout/Reader/XLSX/Helper/CellHelper.php
@@ -10,7 +10,10 @@
  */
 class CellHelper
 {
-    // Using ord() is super slow... Using a pre-computed hash table instead.
+    /**
+     * Using ord() is super slow... Using a pre-computed hash table instead.
+     * @var array<string, int>
+     */
     private static $columnLetterToIndexMapping = [
         'A' => 0, 'B' => 1, 'C' => 2, 'D' => 3, 'E' => 4, 'F' => 5, 'G' => 6,
         'H' => 7, 'I' => 8, 'J' => 9, 'K' => 10, 'L' => 11, 'M' => 12, 'N' => 13,
diff --git a/src/Spout/Reader/XLSX/Helper/CellValueFormatter.php b/src/Spout/Reader/XLSX/Helper/CellValueFormatter.php
index ec8368170..49bc72c1f 100644
--- a/src/Spout/Reader/XLSX/Helper/CellValueFormatter.php
+++ b/src/Spout/Reader/XLSX/Helper/CellValueFormatter.php
@@ -66,7 +66,7 @@ public function __construct($sharedStringsManager, $styleManager, $shouldFormatD
     /**
      * Returns the (unescaped) correctly marshalled, cell value associated to the given XML node.
      *
-     * @param \DOMNode $node
+     * @param \DOMElement $node
      * @throws InvalidValueException If the value is not valid
      * @return string|int|float|bool|\DateTime The value associated with the cell
      */
@@ -102,7 +102,7 @@ public function extractAndFormatNodeValue($node)
     /**
      * Returns the cell's string value from a node's nested value node
      *
-     * @param \DOMNode $node
+     * @param \DOMElement $node
      * @return string The value associated with the cell
      */
     protected function getVNodeValue($node)
@@ -117,7 +117,7 @@ protected function getVNodeValue($node)
     /**
      * Returns the cell String value where string is inline.
      *
-     * @param \DOMNode $node
+     * @param \DOMElement $node
      * @return string The value associated with the cell
      */
     protected function formatInlineStringCellValue($node)
@@ -207,6 +207,7 @@ protected function formatNumericCellValue($nodeValue, $cellStyleId)
     protected function formatExcelTimestampValue($nodeValue, $cellStyleId)
     {
         if ($this->isValidTimestampValue($nodeValue)) {
+            /** @var \DateTime $cellValue */
             $cellValue = $this->formatExcelTimestampValueAsDateTimeValue($nodeValue, $cellStyleId);
         } else {
             throw new InvalidValueException($nodeValue);
@@ -248,6 +249,7 @@ protected function formatExcelTimestampValueAsDateTimeValue($nodeValue, $cellSty
         $timeRemainder = \fmod($nodeValue, 1);
         $secondsRemainder = \round($timeRemainder * self::NUM_SECONDS_IN_ONE_DAY, 0);
 
+        /** @var \DateTime $dateObj */
         $dateObj = \DateTime::createFromFormat('|Y-m-d', $baseDate);
         $dateObj->modify('+' . $daysSinceBaseDate . 'days');
         $dateObj->modify('+' . $secondsRemainder . 'seconds');
diff --git a/src/Spout/Reader/XLSX/Helper/DateFormatHelper.php b/src/Spout/Reader/XLSX/Helper/DateFormatHelper.php
index 443578819..0140f6546 100644
--- a/src/Spout/Reader/XLSX/Helper/DateFormatHelper.php
+++ b/src/Spout/Reader/XLSX/Helper/DateFormatHelper.php
@@ -16,7 +16,7 @@ class DateFormatHelper
      * This map is used to replace Excel format characters by their PHP equivalent.
      * Keys should be ordered from longest to smallest.
      *
-     * @var array Mapping between Excel format characters and PHP format characters
+     * @var array<string, array> Mapping between Excel format characters and PHP format characters
      */
     private static $excelDateFormatToPHPDateFormatMapping = [
         self::KEY_GENERAL => [
@@ -104,6 +104,7 @@ public static function toPHPDateFormat($excelDateFormat)
         // For instance, ["Day " dd] should become [\D\a\y\ dd]
         $phpDateFormat = \preg_replace_callback('/"(.+?)"/', function ($matches) {
             $stringToEscape = $matches[1];
+            /** @var string[] $letters */
             $letters = \preg_split('//u', $stringToEscape, -1, PREG_SPLIT_NO_EMPTY);
 
             return '\\' . \implode('\\', $letters);
diff --git a/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/CachingStrategyFactory.php b/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/CachingStrategyFactory.php
index 1c00a52d6..95bb33cd1 100644
--- a/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/CachingStrategyFactory.php
+++ b/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/CachingStrategyFactory.php
@@ -98,7 +98,7 @@ protected function isInMemoryStrategyUsageSafe($sharedStringsUniqueCount)
     /**
      * Returns the PHP "memory_limit" in Kilobytes
      *
-     * @return float
+     * @return int
      */
     protected function getMemoryLimitInKB()
     {
@@ -133,6 +133,9 @@ protected function getMemoryLimitInKB()
      */
     protected function getMemoryLimitFromIni()
     {
-        return \ini_get('memory_limit');
+        /** @var string $memoryLimit */
+        $memoryLimit = \ini_get('memory_limit');
+
+        return $memoryLimit;
     }
 }
diff --git a/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/FileBasedStrategy.php b/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/FileBasedStrategy.php
index 0e743b492..d209d4045 100644
--- a/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/FileBasedStrategy.php
+++ b/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/FileBasedStrategy.php
@@ -32,7 +32,7 @@ class FileBasedStrategy implements CachingStrategyInterface
      */
     protected $maxNumStringsPerTempFile;
 
-    /** @var resource Pointer to the last temp file a shared string was written to */
+    /** @var resource|null Pointer to the last temp file a shared string was written to */
     protected $tempFilePointer;
 
     /**
@@ -42,7 +42,7 @@ class FileBasedStrategy implements CachingStrategyInterface
     protected $inMemoryTempFilePath;
 
     /**
-     * @var array Contents of the temporary file that was last read
+     * @var array<string> Contents of the temporary file that was last read
      * @see CachingStrategyFactory::MAX_NUM_STRINGS_PER_TEMP_FILE
      */
     protected $inMemoryTempFileContents;
@@ -78,7 +78,9 @@ public function addStringForIndex($sharedString, $sharedStringIndex)
             if ($this->tempFilePointer) {
                 $this->globalFunctionsHelper->fclose($this->tempFilePointer);
             }
-            $this->tempFilePointer = $this->globalFunctionsHelper->fopen($tempFilePath, 'w');
+            /** @var resource $tempFilePointer */
+            $tempFilePointer = $this->globalFunctionsHelper->fopen($tempFilePath, 'w');
+            $this->tempFilePointer = $tempFilePointer;
         }
 
         // The shared string retrieval logic expects each cell data to be on one line only
diff --git a/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/InMemoryStrategy.php b/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/InMemoryStrategy.php
index a16d4de50..59d0d8020 100644
--- a/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/InMemoryStrategy.php
+++ b/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/InMemoryStrategy.php
@@ -12,7 +12,7 @@
  */
 class InMemoryStrategy implements CachingStrategyInterface
 {
-    /** @var \SplFixedArray Array used to cache the shared strings */
+    /** @var \SplFixedArray<string> Array used to cache the shared strings */
     protected $inMemoryCache;
 
     /** @var bool Whether the cache has been closed */
diff --git a/src/Spout/Reader/XLSX/Manager/SharedStringsManager.php b/src/Spout/Reader/XLSX/Manager/SharedStringsManager.php
index 81b4ba299..0da6e54e4 100644
--- a/src/Spout/Reader/XLSX/Manager/SharedStringsManager.php
+++ b/src/Spout/Reader/XLSX/Manager/SharedStringsManager.php
@@ -46,7 +46,7 @@ class SharedStringsManager
     /** @var CachingStrategyFactory Factory to create shared strings caching strategies */
     protected $cachingStrategyFactory;
 
-    /** @var CachingStrategyInterface The best caching strategy for storing shared strings */
+    /** @var CachingStrategyInterface|null The best caching strategy for storing shared strings */
     protected $cachingStrategy;
 
     /**
@@ -179,6 +179,7 @@ protected function processSharedStringsItem($xmlReader, $sharedStringIndex)
         $sharedStringValue = '';
 
         // NOTE: expand() will automatically decode all XML entities of the child nodes
+        /** @var \DOMElement $siNode */
         $siNode = $xmlReader->expand();
         $textNodes = $siNode->getElementsByTagName(self::XML_NODE_T);
 
diff --git a/src/Spout/Reader/XLSX/Manager/SheetManager.php b/src/Spout/Reader/XLSX/Manager/SheetManager.php
index e7b41e010..d6b5126b2 100644
--- a/src/Spout/Reader/XLSX/Manager/SheetManager.php
+++ b/src/Spout/Reader/XLSX/Manager/SheetManager.php
@@ -54,7 +54,7 @@ class SheetManager
     /** @var \Box\Spout\Common\Helper\Escaper\XLSX Used to unescape XML data */
     protected $escaper;
 
-    /** @var array List of sheets */
+    /** @var array<Sheet> List of sheets */
     protected $sheets;
 
     /** @var int Index of the sheet currently read */
diff --git a/src/Spout/Reader/XLSX/Manager/StyleManager.php b/src/Spout/Reader/XLSX/Manager/StyleManager.php
index d5db0d588..f51771f97 100644
--- a/src/Spout/Reader/XLSX/Manager/StyleManager.php
+++ b/src/Spout/Reader/XLSX/Manager/StyleManager.php
@@ -28,7 +28,7 @@ class StyleManager
 
     /**
      * @see https://msdn.microsoft.com/en-us/library/ff529597(v=office.12).aspx
-     * @var array Mapping between built-in numFmtId and the associated format - for dates only
+     * @var array<int, string> Mapping between built-in numFmtId and the associated format - for dates only
      */
     protected static $builtinNumFmtIdToNumFormatMapping = [
         14 => 'm/d/yyyy', // @NOTE: ECMA spec is 'mm-dd-yy'
@@ -57,16 +57,16 @@ class StyleManager
     /** @var InternalEntityFactory Factory to create entities */
     protected $entityFactory;
 
-    /** @var array Array containing the IDs of built-in number formats indicating a date */
+    /** @var int[]|string[] Array containing the IDs of built-in number formats indicating a date */
     protected $builtinNumFmtIdIndicatingDates;
 
-    /** @var array Array containing a mapping NUM_FMT_ID => FORMAT_CODE */
+    /** @var array<int,string> Array containing a mapping NUM_FMT_ID => FORMAT_CODE */
     protected $customNumberFormats;
 
-    /** @var array Array containing a mapping STYLE_ID => [STYLE_ATTRIBUTES] */
+    /** @var array<mixed> Array containing a mapping STYLE_ID => [STYLE_ATTRIBUTES] */
     protected $stylesAttributes;
 
-    /** @var array Cache containing a mapping NUM_FMT_ID => IS_DATE_FORMAT. Used to avoid lots of recalculations */
+    /** @var array<int, bool> Cache containing a mapping NUM_FMT_ID => IS_DATE_FORMAT. Used to avoid lots of recalculations */
     protected $numFmtIdToIsDateFormatCache = [];
 
     /**
@@ -189,7 +189,7 @@ protected function extractStyleAttributes($xmlReader)
     }
 
     /**
-     * @return array The custom number formats
+     * @return array<int,string>  The custom number formats
      */
     protected function getCustomNumberFormats()
     {
@@ -201,7 +201,7 @@ protected function getCustomNumberFormats()
     }
 
     /**
-     * @return array The styles attributes
+     * @return array<mixed>  The styles attributes
      */
     protected function getStylesAttributes()
     {
@@ -213,7 +213,7 @@ protected function getStylesAttributes()
     }
 
     /**
-     * @param array $styleAttributes Array containing the style attributes (2 keys: "applyNumberFormat" and "numFmtId")
+     * @param array<string, mixed> $styleAttributes Array containing the style attributes (2 keys: "applyNumberFormat" and "numFmtId")
      * @return bool Whether the style with the given attributes indicates that the number is a date
      */
     protected function doesStyleIndicateDate($styleAttributes)
diff --git a/src/Spout/Reader/XLSX/Manager/WorkbookRelationshipsManager.php b/src/Spout/Reader/XLSX/Manager/WorkbookRelationshipsManager.php
index 0a9f6f51a..f9fb82ca8 100644
--- a/src/Spout/Reader/XLSX/Manager/WorkbookRelationshipsManager.php
+++ b/src/Spout/Reader/XLSX/Manager/WorkbookRelationshipsManager.php
@@ -34,7 +34,7 @@ class WorkbookRelationshipsManager
     /** @var InternalEntityFactory Factory to create entities */
     private $entityFactory;
 
-    /** @var array Cache of the already read workbook relationships: [TYPE] => [FILE_NAME] */
+    /** @var array<string, string> Cache of the already read workbook relationships: [TYPE] => [FILE_NAME] */
     private $cachedWorkbookRelationships;
 
     /**
@@ -112,7 +112,7 @@ public function getStylesXMLFilePath()
      * It caches the result so that the file is read only once.
      *
      * @throws \Box\Spout\Common\Exception\IOException If workbook.xml.rels can't be read
-     * @return array
+     * @return array<string, string>
      */
     private function getWorkbookRelationships()
     {
diff --git a/src/Spout/Reader/XLSX/Reader.php b/src/Spout/Reader/XLSX/Reader.php
index 689f6e2f1..c64f550af 100644
--- a/src/Spout/Reader/XLSX/Reader.php
+++ b/src/Spout/Reader/XLSX/Reader.php
@@ -20,10 +20,10 @@ class Reader extends ReaderAbstract
     /** @var ManagerFactory */
     protected $managerFactory;
 
-    /** @var \ZipArchive */
+    /** @var \ZipArchive|null */
     protected $zip;
 
-    /** @var \Box\Spout\Reader\XLSX\Manager\SharedStringsManager Manages shared strings */
+    /** @var \Box\Spout\Reader\XLSX\Manager\SharedStringsManager|null Manages shared strings */
     protected $sharedStringsManager;
 
     /** @var SheetIterator To iterator over the XLSX sheets */
diff --git a/src/Spout/Reader/XLSX/RowIterator.php b/src/Spout/Reader/XLSX/RowIterator.php
index a54b8b192..b543c1865 100644
--- a/src/Spout/Reader/XLSX/RowIterator.php
+++ b/src/Spout/Reader/XLSX/RowIterator.php
@@ -17,6 +17,7 @@
 
 /**
  * Class RowIterator
+ * @implements IteratorInterface<Row>
  */
 class RowIterator implements IteratorInterface
 {
@@ -276,6 +277,7 @@ protected function processCellStartingNode($xmlReader)
         $currentColumnIndex = $this->getColumnIndex($xmlReader);
 
         // NOTE: expand() will automatically decode all XML entities of the child nodes
+        /** @var \DOMElement $node */
         $node = $xmlReader->expand();
         $cell = $this->getCell($node);
 
@@ -352,7 +354,7 @@ protected function getColumnIndex($xmlReader)
     /**
      * Returns the cell with (unescaped) correctly marshalled, cell value associated to the given XML node.
      *
-     * @param \DOMNode $node
+     * @param \DOMElement $node
      * @return Cell The cell set with the associated with the cell
      */
     protected function getCell($node)
diff --git a/src/Spout/Reader/XLSX/SheetIterator.php b/src/Spout/Reader/XLSX/SheetIterator.php
index 81f481c3c..fdd393343 100644
--- a/src/Spout/Reader/XLSX/SheetIterator.php
+++ b/src/Spout/Reader/XLSX/SheetIterator.php
@@ -9,6 +9,7 @@
 /**
  * Class SheetIterator
  * Iterate over XLSX sheet.
+ * @implements IteratorInterface<Sheet>
  */
 class SheetIterator implements IteratorInterface
 {
diff --git a/src/Spout/Writer/Common/Creator/WriterEntityFactory.php b/src/Spout/Writer/Common/Creator/WriterEntityFactory.php
index 8e43c31e3..cc1b0fab4 100644
--- a/src/Spout/Writer/Common/Creator/WriterEntityFactory.php
+++ b/src/Spout/Writer/Common/Creator/WriterEntityFactory.php
@@ -5,7 +5,6 @@
 use Box\Spout\Common\Entity\Cell;
 use Box\Spout\Common\Entity\Row;
 use Box\Spout\Common\Entity\Style\Style;
-use Box\Spout\Common\Exception\UnsupportedTypeException;
 use Box\Spout\Common\Type;
 use Box\Spout\Writer\WriterInterface;
 
@@ -46,11 +45,10 @@ public static function createWriterFromFile(string $path)
      */
     public static function createCSVWriter()
     {
-        try {
-            return WriterFactory::createFromType(Type::CSV);
-        } catch (UnsupportedTypeException $e) {
-            // should never happen
-        }
+        /** @var \Box\Spout\Writer\CSV\Writer $csvWriter */
+        $csvWriter = WriterFactory::createFromType(Type::CSV);
+
+        return $csvWriter;
     }
 
     /**
@@ -60,11 +58,10 @@ public static function createCSVWriter()
      */
     public static function createXLSXWriter()
     {
-        try {
-            return WriterFactory::createFromType(Type::XLSX);
-        } catch (UnsupportedTypeException $e) {
-            // should never happen
-        }
+        /** @var \Box\Spout\Writer\XLSX\Writer $xlsxWriter */
+        $xlsxWriter = WriterFactory::createFromType(Type::XLSX);
+
+        return $xlsxWriter;
     }
 
     /**
@@ -74,11 +71,10 @@ public static function createXLSXWriter()
      */
     public static function createODSWriter()
     {
-        try {
-            return WriterFactory::createFromType(Type::ODS);
-        } catch (UnsupportedTypeException $e) {
-            // should never happen
-        }
+        /** @var \Box\Spout\Writer\ODS\Writer $odsWriter */
+        $odsWriter = WriterFactory::createFromType(Type::ODS);
+
+        return $odsWriter;
     }
 
     /**
@@ -92,7 +88,7 @@ public static function createRow(array $cells = [], Style $rowStyle = null)
     }
 
     /**
-     * @param array $cellValues
+     * @param array<mixed> $cellValues
      * @param Style|null $rowStyle
      * @return Row
      */
diff --git a/src/Spout/Writer/Common/Entity/Workbook.php b/src/Spout/Writer/Common/Entity/Workbook.php
index dd182199d..1a803b388 100644
--- a/src/Spout/Writer/Common/Entity/Workbook.php
+++ b/src/Spout/Writer/Common/Entity/Workbook.php
@@ -33,7 +33,7 @@ public function getWorksheets()
     /**
      * @param Worksheet[] $worksheets
      */
-    public function setWorksheets($worksheets)
+    public function setWorksheets($worksheets) : void
     {
         $this->worksheets = $worksheets;
     }
diff --git a/src/Spout/Writer/Common/Entity/Worksheet.php b/src/Spout/Writer/Common/Entity/Worksheet.php
index 74c4976f0..9506d00a4 100644
--- a/src/Spout/Writer/Common/Entity/Worksheet.php
+++ b/src/Spout/Writer/Common/Entity/Worksheet.php
@@ -11,10 +11,10 @@ class Worksheet
     /** @var string Path to the XML file that will contain the sheet data */
     private $filePath;
 
-    /** @var resource Pointer to the sheet data file (e.g. xl/worksheets/sheet1.xml) */
+    /** @var resource|null Pointer to the sheet data file (e.g. xl/worksheets/sheet1.xml) */
     private $filePointer;
 
-    /** @var Sheet The "external" sheet */
+    /** @var Sheet|null The "external" sheet */
     private $externalSheet;
 
     /** @var int Maximum number of columns among all the written rows */
@@ -57,7 +57,7 @@ public function getFilePointer()
     /**
      * @param resource $filePointer
      */
-    public function setFilePointer($filePointer)
+    public function setFilePointer($filePointer) : void
     {
         $this->filePointer = $filePointer;
     }
@@ -81,7 +81,7 @@ public function getMaxNumColumns()
     /**
      * @param int $maxNumColumns
      */
-    public function setMaxNumColumns($maxNumColumns)
+    public function setMaxNumColumns($maxNumColumns) : void
     {
         $this->maxNumColumns = $maxNumColumns;
     }
@@ -97,7 +97,7 @@ public function getLastWrittenRowIndex()
     /**
      * @param int $lastWrittenRowIndex
      */
-    public function setLastWrittenRowIndex($lastWrittenRowIndex)
+    public function setLastWrittenRowIndex($lastWrittenRowIndex) : void
     {
         $this->lastWrittenRowIndex = $lastWrittenRowIndex;
     }
diff --git a/src/Spout/Writer/Common/Helper/CellHelper.php b/src/Spout/Writer/Common/Helper/CellHelper.php
index afe3c7126..9032b7df9 100644
--- a/src/Spout/Writer/Common/Helper/CellHelper.php
+++ b/src/Spout/Writer/Common/Helper/CellHelper.php
@@ -8,7 +8,7 @@
  */
 class CellHelper
 {
-    /** @var array Cache containing the mapping column index => column letters */
+    /** @var array<int, string> Cache containing the mapping column index => column letters */
     private static $columnIndexToColumnLettersCache = [];
 
     /**
diff --git a/src/Spout/Writer/Common/Helper/ZipHelper.php b/src/Spout/Writer/Common/Helper/ZipHelper.php
index 7a250c86a..566b92738 100644
--- a/src/Spout/Writer/Common/Helper/ZipHelper.php
+++ b/src/Spout/Writer/Common/Helper/ZipHelper.php
@@ -181,9 +181,13 @@ protected function shouldSkipFile($zip, $itemLocalPath, $existingFileMode)
      */
     protected function getNormalizedRealPath($path)
     {
+        /** @var string $realPath */
         $realPath = \realpath($path);
 
-        return \str_replace(DIRECTORY_SEPARATOR, '/', $realPath);
+        /** @var string $normalized */
+        $normalized = \str_replace(DIRECTORY_SEPARATOR, '/', $realPath);
+
+        return $normalized;
     }
 
     /**
@@ -210,6 +214,7 @@ public function closeArchiveAndCopyToStream($zip, $streamPointer)
      */
     protected function copyZipToStream($zipFilePath, $pointer)
     {
+        /** @var resource $zipFilePointer */
         $zipFilePointer = \fopen($zipFilePath, 'r');
         \stream_copy_to_stream($zipFilePointer, $pointer);
         \fclose($zipFilePointer);
diff --git a/src/Spout/Writer/Common/Manager/SheetManager.php b/src/Spout/Writer/Common/Manager/SheetManager.php
index d1d4e6a04..249292a9e 100644
--- a/src/Spout/Writer/Common/Manager/SheetManager.php
+++ b/src/Spout/Writer/Common/Manager/SheetManager.php
@@ -15,10 +15,10 @@ class SheetManager
     /** Sheet name should not exceed 31 characters */
     const MAX_LENGTH_SHEET_NAME = 31;
 
-    /** @var array Invalid characters that cannot be contained in the sheet name */
+    /** @var array<string> Invalid characters that cannot be contained in the sheet name */
     private static $INVALID_CHARACTERS_IN_SHEET_NAME = ['\\', '/', '?', '*', ':', '[', ']'];
 
-    /** @var array Associative array [WORKBOOK_ID] => [[SHEET_INDEX] => [SHEET_NAME]] keeping track of sheets' name to enforce uniqueness per workbook */
+    /** @var array<string, array> Associative array [WORKBOOK_ID] => [[SHEET_INDEX] => [SHEET_NAME]] keeping track of sheets' name to enforce uniqueness per workbook */
     private static $SHEETS_NAME_USED = [];
 
     /** @var StringHelper */
@@ -126,7 +126,7 @@ private function isNameUnique($name, Sheet $sheet)
     }
 
     /**
-     * @param int $workbookId Workbook ID associated to a Sheet
+     * @param string $workbookId Workbook ID associated to a Sheet
      * @return void
      */
     public function markWorkbookIdAsUsed($workbookId)
diff --git a/src/Spout/Writer/Common/Manager/Style/PossiblyUpdatedStyle.php b/src/Spout/Writer/Common/Manager/Style/PossiblyUpdatedStyle.php
index 6ccaa29d9..b8087183f 100644
--- a/src/Spout/Writer/Common/Manager/Style/PossiblyUpdatedStyle.php
+++ b/src/Spout/Writer/Common/Manager/Style/PossiblyUpdatedStyle.php
@@ -11,7 +11,14 @@
  */
 class PossiblyUpdatedStyle
 {
+    /**
+     * @var Style
+     */
     private $style;
+
+    /**
+     * @var bool
+     */
     private $isUpdated;
 
     public function __construct(Style $style, bool $isUpdated)
diff --git a/src/Spout/Writer/Common/Manager/Style/StyleRegistry.php b/src/Spout/Writer/Common/Manager/Style/StyleRegistry.php
index 6b439a75d..5a7e5c1ef 100644
--- a/src/Spout/Writer/Common/Manager/Style/StyleRegistry.php
+++ b/src/Spout/Writer/Common/Manager/Style/StyleRegistry.php
@@ -10,10 +10,10 @@
  */
 class StyleRegistry
 {
-    /** @var array [SERIALIZED_STYLE] => [STYLE_ID] mapping table, keeping track of the registered styles */
+    /** @var array<string, int> [SERIALIZED_STYLE] => [STYLE_ID] mapping table, keeping track of the registered styles */
     protected $serializedStyleToStyleIdMappingTable = [];
 
-    /** @var array [STYLE_ID] => [STYLE] mapping table, keeping track of the registered styles */
+    /** @var array<int, Style> [STYLE_ID] => [STYLE] mapping table, keeping track of the registered styles */
     protected $styleIdToStyleMappingTable = [];
 
     /**
diff --git a/src/Spout/Writer/Common/Manager/WorkbookManagerAbstract.php b/src/Spout/Writer/Common/Manager/WorkbookManagerAbstract.php
index 653778c70..07f01877d 100644
--- a/src/Spout/Writer/Common/Manager/WorkbookManagerAbstract.php
+++ b/src/Spout/Writer/Common/Manager/WorkbookManagerAbstract.php
@@ -265,6 +265,7 @@ private function addRowToWorksheet(Worksheet $worksheet, Row $row)
 
     /**
      * @param Row $row
+     * @return void
      */
     private function applyDefaultRowStyle(Row $row)
     {
diff --git a/src/Spout/Writer/Exception/Border/InvalidNameException.php b/src/Spout/Writer/Exception/Border/InvalidNameException.php
index c975d4fc4..93514c37f 100644
--- a/src/Spout/Writer/Exception/Border/InvalidNameException.php
+++ b/src/Spout/Writer/Exception/Border/InvalidNameException.php
@@ -7,6 +7,9 @@
 
 class InvalidNameException extends WriterException
 {
+    /**
+     * @param string $name
+     */
     public function __construct($name)
     {
         $msg = '%s is not a valid name identifier for a border. Valid identifiers are: %s.';
diff --git a/src/Spout/Writer/Exception/Border/InvalidStyleException.php b/src/Spout/Writer/Exception/Border/InvalidStyleException.php
index 2d1d78c86..ed02bb826 100644
--- a/src/Spout/Writer/Exception/Border/InvalidStyleException.php
+++ b/src/Spout/Writer/Exception/Border/InvalidStyleException.php
@@ -7,6 +7,9 @@
 
 class InvalidStyleException extends WriterException
 {
+    /**
+     * @param string $name
+     */
     public function __construct($name)
     {
         $msg = '%s is not a valid style identifier for a border. Valid identifiers are: %s.';
diff --git a/src/Spout/Writer/Exception/Border/InvalidWidthException.php b/src/Spout/Writer/Exception/Border/InvalidWidthException.php
index 790ddc2cb..b61257c54 100644
--- a/src/Spout/Writer/Exception/Border/InvalidWidthException.php
+++ b/src/Spout/Writer/Exception/Border/InvalidWidthException.php
@@ -7,6 +7,9 @@
 
 class InvalidWidthException extends WriterException
 {
+    /**
+     * @param string $name
+     */
     public function __construct($name)
     {
         $msg = '%s is not a valid width identifier for a border. Valid identifiers are: %s.';
diff --git a/src/Spout/Writer/ODS/Creator/HelperFactory.php b/src/Spout/Writer/ODS/Creator/HelperFactory.php
index e6aa64539..cdc5a6ab9 100644
--- a/src/Spout/Writer/ODS/Creator/HelperFactory.php
+++ b/src/Spout/Writer/ODS/Creator/HelperFactory.php
@@ -30,7 +30,7 @@ public function createSpecificFileSystemHelper(OptionsManagerInterface $optionsM
     }
 
     /**
-     * @param $entityFactory
+     * @param InternalEntityFactory $entityFactory
      * @return ZipHelper
      */
     private function createZipHelper($entityFactory)
diff --git a/src/Spout/Writer/ODS/Helper/BorderHelper.php b/src/Spout/Writer/ODS/Helper/BorderHelper.php
index 34886acf4..1fe124a14 100644
--- a/src/Spout/Writer/ODS/Helper/BorderHelper.php
+++ b/src/Spout/Writer/ODS/Helper/BorderHelper.php
@@ -24,7 +24,7 @@ class BorderHelper
     /**
      * Width mappings
      *
-     * @var array
+     * @var array<string, string>
      */
     protected static $widthMap = [
         Border::WIDTH_THIN   => '0.75pt',
@@ -35,7 +35,7 @@ class BorderHelper
     /**
      * Style mapping
      *
-     * @var array
+     * @var array<string, string>
      */
     protected static $styleMap = [
         Border::STYLE_SOLID  => 'solid',
diff --git a/src/Spout/Writer/ODS/Helper/FileSystemHelper.php b/src/Spout/Writer/ODS/Helper/FileSystemHelper.php
index 5598bb4a5..172ce19ce 100644
--- a/src/Spout/Writer/ODS/Helper/FileSystemHelper.php
+++ b/src/Spout/Writer/ODS/Helper/FileSystemHelper.php
@@ -210,6 +210,7 @@ public function createContentFile($worksheetManager, $styleManager, $worksheets)
 
         // Append sheets content to "content.xml"
         $contentXmlFilePath = $this->rootFolder . '/' . self::CONTENT_XML_FILE_NAME;
+        /** @var resource $contentXmlHandle */
         $contentXmlHandle = \fopen($contentXmlFilePath, 'a');
 
         foreach ($worksheets as $worksheet) {
@@ -241,6 +242,7 @@ public function createContentFile($worksheetManager, $styleManager, $worksheets)
      */
     protected function copyFileContentsToTarget($sourceFilePath, $targetResource)
     {
+        /** @var resource $sourceHandle */
         $sourceHandle = \fopen($sourceFilePath, 'r');
         \stream_copy_to_stream($sourceHandle, $targetResource);
         \fclose($sourceHandle);
diff --git a/src/Spout/Writer/ODS/Manager/Style/StyleRegistry.php b/src/Spout/Writer/ODS/Manager/Style/StyleRegistry.php
index 42484f292..47ca61cb2 100644
--- a/src/Spout/Writer/ODS/Manager/Style/StyleRegistry.php
+++ b/src/Spout/Writer/ODS/Manager/Style/StyleRegistry.php
@@ -10,7 +10,7 @@
  */
 class StyleRegistry extends \Box\Spout\Writer\Common\Manager\Style\StyleRegistry
 {
-    /** @var array [FONT_NAME] => [] Map whose keys contain all the fonts used */
+    /** @var array<string, bool> [FONT_NAME] => [] Map whose keys contain all the fonts used */
     protected $usedFontsSet = [];
 
     /**
diff --git a/src/Spout/Writer/ODS/Manager/WorkbookManager.php b/src/Spout/Writer/ODS/Manager/WorkbookManager.php
index 77c5f9037..bda1b16c1 100644
--- a/src/Spout/Writer/ODS/Manager/WorkbookManager.php
+++ b/src/Spout/Writer/ODS/Manager/WorkbookManager.php
@@ -16,6 +16,7 @@ class WorkbookManager extends WorkbookManagerAbstract
     /**
      * Maximum number of rows a ODS sheet can contain
      * @see https://ask.libreoffice.org/en/question/8631/upper-limit-to-number-of-rows-in-calc/
+     * @var int
      */
     protected static $maxRowsPerWorksheet = 1048576;
 
diff --git a/src/Spout/Writer/ODS/Manager/WorksheetManager.php b/src/Spout/Writer/ODS/Manager/WorksheetManager.php
index 7d7cb0ebb..11b396e8b 100644
--- a/src/Spout/Writer/ODS/Manager/WorksheetManager.php
+++ b/src/Spout/Writer/ODS/Manager/WorksheetManager.php
@@ -62,6 +62,7 @@ public function __construct(
      */
     public function startSheet(Worksheet $worksheet)
     {
+        /** @var resource $sheetFilePointer */
         $sheetFilePointer = \fopen($worksheet->getFilePath(), 'w');
         $this->throwIfSheetFilePointerIsNotAvailable($sheetFilePointer);
 
@@ -86,7 +87,7 @@ private function throwIfSheetFilePointerIsNotAvailable($sheetFilePointer)
      * Returns the table XML root node as string.
      *
      * @param Worksheet $worksheet
-     * @return string <table> node as string
+     * @return string "<table> node as string"
      */
     public function getTableElementStartAsString(Worksheet $worksheet)
     {
diff --git a/src/Spout/Writer/WriterAbstract.php b/src/Spout/Writer/WriterAbstract.php
index 36a583fe6..104539fd1 100644
--- a/src/Spout/Writer/WriterAbstract.php
+++ b/src/Spout/Writer/WriterAbstract.php
@@ -24,7 +24,7 @@ abstract class WriterAbstract implements WriterInterface
     /** @var string Path to the output file */
     protected $outputFilePath;
 
-    /** @var resource Pointer to the file/stream we will write to */
+    /** @var resource|null Pointer to the file/stream we will write to */
     protected $filePointer;
 
     /** @var bool Indicates whether the writer has been opened or not */
@@ -99,7 +99,10 @@ public function openToFile($outputFilePath)
     {
         $this->outputFilePath = $outputFilePath;
 
-        $this->filePointer = $this->globalFunctionsHelper->fopen($this->outputFilePath, 'wb+');
+        /** @var resource $filePointer */
+        $filePointer = $this->globalFunctionsHelper->fopen($this->outputFilePath, 'wb+');
+
+        $this->filePointer = $filePointer;
         $this->throwIfFilePointerIsNotAvailable();
 
         $this->openWriter();
@@ -115,8 +118,10 @@ public function openToFile($outputFilePath)
     public function openToBrowser($outputFileName)
     {
         $this->outputFilePath = $this->globalFunctionsHelper->basename($outputFileName);
+        /** @var resource $filePointer */
+        $filePointer = $this->globalFunctionsHelper->fopen('php://output', 'w');
 
-        $this->filePointer = $this->globalFunctionsHelper->fopen('php://output', 'w');
+        $this->filePointer = $filePointer;
         $this->throwIfFilePointerIsNotAvailable();
 
         // Clear any previous output (otherwise the generated file will be corrupted)
diff --git a/src/Spout/Writer/WriterMultiSheetsAbstract.php b/src/Spout/Writer/WriterMultiSheetsAbstract.php
index 8170b679c..6128ec374 100644
--- a/src/Spout/Writer/WriterMultiSheetsAbstract.php
+++ b/src/Spout/Writer/WriterMultiSheetsAbstract.php
@@ -25,7 +25,7 @@ abstract class WriterMultiSheetsAbstract extends WriterAbstract
     /** @var ManagerFactoryInterface */
     private $managerFactory;
 
-    /** @var WorkbookManagerInterface */
+    /** @var WorkbookManagerInterface|null */
     private $workbookManager;
 
     /**
@@ -143,7 +143,7 @@ public function setCurrentSheet($sheet)
      */
     protected function throwIfWorkbookIsNotAvailable()
     {
-        if (!$this->workbookManager->getWorkbook()) {
+        if ($this->workbookManager === null) {
             throw new WriterNotOpenedException('The writer must be opened before performing this action.');
         }
     }
diff --git a/src/Spout/Writer/XLSX/Helper/BorderHelper.php b/src/Spout/Writer/XLSX/Helper/BorderHelper.php
index ed202cb63..59d738604 100644
--- a/src/Spout/Writer/XLSX/Helper/BorderHelper.php
+++ b/src/Spout/Writer/XLSX/Helper/BorderHelper.php
@@ -7,6 +7,9 @@
 
 class BorderHelper
 {
+    /**
+     * @var array<string, array>
+     */
     public static $xlsxStyleMap = [
         Border::STYLE_SOLID => [
             Border::WIDTH_THIN   => 'thin',
diff --git a/src/Spout/Writer/XLSX/Manager/SharedStringsManager.php b/src/Spout/Writer/XLSX/Manager/SharedStringsManager.php
index b0a5b48d6..76703fb8e 100644
--- a/src/Spout/Writer/XLSX/Manager/SharedStringsManager.php
+++ b/src/Spout/Writer/XLSX/Manager/SharedStringsManager.php
@@ -24,7 +24,7 @@ class SharedStringsManager
      */
     const DEFAULT_STRINGS_COUNT_PART = 'count="9999999999999" uniqueCount="9999999999999"';
 
-    /** @var resource Pointer to the sharedStrings.xml file */
+    /** @var resource|false Pointer to the sharedStrings.xml file */
     protected $sharedStringsFilePointer;
 
     /** @var int Number of shared strings already written */
@@ -40,13 +40,17 @@ class SharedStringsManager
     public function __construct($xlFolder, $stringsEscaper)
     {
         $sharedStringsFilePath = $xlFolder . '/' . self::SHARED_STRINGS_FILE_NAME;
-        $this->sharedStringsFilePointer = \fopen($sharedStringsFilePath, 'w');
+
+        /** @var resource $sharedStringsFilePointer */
+        $sharedStringsFilePointer =\fopen($sharedStringsFilePath, 'w');
+
+        $this->sharedStringsFilePointer = $sharedStringsFilePointer;
 
         $this->throwIfSharedStringsFilePointerIsNotAvailable();
 
         // the headers is split into different parts so that we can fseek and put in the correct count and uniqueCount later
         $header = self::SHARED_STRINGS_XML_FILE_FIRST_PART_HEADER . ' ' . self::DEFAULT_STRINGS_COUNT_PART . '>';
-        \fwrite($this->sharedStringsFilePointer, $header);
+        \fwrite($sharedStringsFilePointer, $header);
 
         $this->stringsEscaper = $stringsEscaper;
     }
@@ -59,7 +63,7 @@ public function __construct($xlFolder, $stringsEscaper)
      */
     protected function throwIfSharedStringsFilePointerIsNotAvailable()
     {
-        if (!$this->sharedStringsFilePointer) {
+        if ($this->sharedStringsFilePointer === false) {
             throw new IOException('Unable to open shared strings file for writing.');
         }
     }
@@ -73,7 +77,10 @@ protected function throwIfSharedStringsFilePointerIsNotAvailable()
      */
     public function writeString($string)
     {
-        \fwrite($this->sharedStringsFilePointer, '<si><t xml:space="preserve">' . $this->stringsEscaper->escape($string) . '</t></si>');
+        /** @var resource $sharedStringsFilePointer */
+        $sharedStringsFilePointer = $this->sharedStringsFilePointer;
+
+        \fwrite($sharedStringsFilePointer, '<si><t xml:space="preserve">' . $this->stringsEscaper->escape($string) . '</t></si>');
         $this->numSharedStrings++;
 
         // Shared string ID is zero-based
diff --git a/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php b/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
index f0ca9d9e6..66059c05f 100644
--- a/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
+++ b/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
@@ -2,6 +2,7 @@
 
 namespace Box\Spout\Writer\XLSX\Manager\Style;
 
+use Box\Spout\Common\Entity\Style\BorderPart;
 use Box\Spout\Common\Entity\Style\Color;
 use Box\Spout\Common\Entity\Style\Style;
 use Box\Spout\Writer\XLSX\Helper\BorderHelper;
@@ -196,7 +197,7 @@ protected function getBordersSectionContent()
 
             foreach ($sortOrder as $partName) {
                 if ($border->hasPart($partName)) {
-                    /** @var $part \Box\Spout\Common\Entity\Style\BorderPart */
+                    /** @var BorderPart $part */
                     $part = $border->getPart($partName);
                     $content .= BorderHelper::serializeBorderPart($part);
                 }
diff --git a/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php b/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php
index 14eb98623..d18d8fd1c 100644
--- a/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php
+++ b/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php
@@ -12,7 +12,7 @@ class StyleRegistry extends \Box\Spout\Writer\Common\Manager\Style\StyleRegistry
 {
     /**
      * @see https://msdn.microsoft.com/en-us/library/ff529597(v=office.12).aspx
-     * @var array Mapping between built-in format and the associated numFmtId
+     * @var array<int|string, int> Mapping between built-in format and the associated numFmtId
      */
     protected static $builtinNumFormatToIdMapping = [
         'General' => 0,
@@ -65,12 +65,12 @@ class StyleRegistry extends \Box\Spout\Writer\Common\Manager\Style\StyleRegistry
     ];
 
     /**
-     * @var array
+     * @var array<string, int>
      */
     protected $registeredFormats = [];
 
     /**
-     * @var array [STYLE_ID] => [FORMAT_ID] maps a style to a format declaration
+     * @var array <int, int> [STYLE_ID] => [FORMAT_ID] maps a style to a format declaration
      */
     protected $styleIdToFormatsMappingTable = [];
 
@@ -84,12 +84,12 @@ class StyleRegistry extends \Box\Spout\Writer\Common\Manager\Style\StyleRegistry
     protected $formatIndex = 164;
 
     /**
-     * @var array
+     * @var array<string, int>
      */
     protected $registeredFills = [];
 
     /**
-     * @var array [STYLE_ID] => [FILL_ID] maps a style to a fill declaration
+     * @var array<int, int> [STYLE_ID] => [FILL_ID] maps a style to a fill declaration
      */
     protected $styleIdToFillMappingTable = [];
 
@@ -102,12 +102,12 @@ class StyleRegistry extends \Box\Spout\Writer\Common\Manager\Style\StyleRegistry
     protected $fillIndex = 2;
 
     /**
-     * @var array
+     * @var array<string, int>
      */
     protected $registeredBorders = [];
 
     /**
-     * @var array [STYLE_ID] => [BORDER_ID] maps a style to a border declaration
+     * @var array<int, int> [STYLE_ID] => [BORDER_ID] maps a style to a border declaration
      */
     protected $styleIdToBorderMappingTable = [];
 
@@ -136,8 +136,9 @@ public function registerStyle(Style $style)
      *
      * @param Style $style
      */
-    protected function registerFormat(Style $style)
+    protected function registerFormat(Style $style) : void
     {
+        /** @var int $styleId */
         $styleId = $style->getId();
 
         $format = $style->getFormat();
@@ -176,8 +177,9 @@ public function getFormatIdForStyleId($styleId)
      *
      * @param Style $style
      */
-    private function registerFill(Style $style)
+    private function registerFill(Style $style) : void
     {
+        /** @var int $styleId */
         $styleId = $style->getId();
 
         // Currently - only solid backgrounds are supported
@@ -219,8 +221,9 @@ public function getFillIdForStyleId($styleId)
      *
      * @param Style $style
      */
-    private function registerBorder(Style $style)
+    private function registerBorder(Style $style) : void
     {
+        /** @var int $styleId */
         $styleId = $style->getId();
 
         if ($style->shouldApplyBorder()) {
@@ -255,7 +258,7 @@ public function getBorderIdForStyleId($styleId)
     }
 
     /**
-     * @return array
+     * @return array<string, int>
      */
     public function getRegisteredFills()
     {
@@ -263,7 +266,7 @@ public function getRegisteredFills()
     }
 
     /**
-     * @return array
+     * @return array<string, int>
      */
     public function getRegisteredBorders()
     {
@@ -271,7 +274,7 @@ public function getRegisteredBorders()
     }
 
     /**
-     * @return array
+     * @return array<string, int>
      */
     public function getRegisteredFormats()
     {
diff --git a/src/Spout/Writer/XLSX/Manager/WorkbookManager.php b/src/Spout/Writer/XLSX/Manager/WorkbookManager.php
index 708208bd5..1b16f9fd3 100644
--- a/src/Spout/Writer/XLSX/Manager/WorkbookManager.php
+++ b/src/Spout/Writer/XLSX/Manager/WorkbookManager.php
@@ -14,6 +14,7 @@
 class WorkbookManager extends WorkbookManagerAbstract
 {
     /**
+     * @var int
      * Maximum number of rows a XLSX sheet can contain
      * @see http://office.microsoft.com/en-us/excel-help/excel-specifications-and-limits-HP010073849.aspx
      */
diff --git a/src/Spout/Writer/XLSX/Manager/WorksheetManager.php b/src/Spout/Writer/XLSX/Manager/WorksheetManager.php
index 61b93a176..14ffecb03 100644
--- a/src/Spout/Writer/XLSX/Manager/WorksheetManager.php
+++ b/src/Spout/Writer/XLSX/Manager/WorksheetManager.php
@@ -108,6 +108,7 @@ public function getSharedStringsManager()
      */
     public function startSheet(Worksheet $worksheet)
     {
+        /** @var resource $sheetFilePointer */
         $sheetFilePointer = \fopen($worksheet->getFilePath(), 'w');
         $this->throwIfSheetFilePointerIsNotAvailable($sheetFilePointer);
 
diff --git a/tests/Spout/Common/Entity/RowTest.php b/tests/Spout/Common/Entity/RowTest.php
index 9aa12f607..d734bcae8 100644
--- a/tests/Spout/Common/Entity/RowTest.php
+++ b/tests/Spout/Common/Entity/RowTest.php
@@ -3,23 +3,30 @@
 namespace Box\Spout\Common\Entity;
 
 use Box\Spout\Common\Entity\Style\Style;
+use PHPUnit\Framework\MockObject\MockObject;
 
 class RowTest extends \PHPUnit\Framework\TestCase
 {
     /**
-     * @return \PHPUnit_Framework_MockObject_MockObject|Style
+     * @return Style&MockObject
      */
     private function getStyleMock()
     {
-        return $this->createMock(Style::class);
+        /** @var Style&MockObject $styleMock */
+        $styleMock = $this->createMock(Style::class);
+
+        return $styleMock;
     }
 
     /**
-     * @return \PHPUnit_Framework_MockObject_MockObject|Cell
+     * @return Cell&MockObject
      */
     private function getCellMock()
     {
-        return $this->createMock(Cell::class);
+        /** @var Cell&MockObject $cellMock */
+        $cellMock = $this->createMock(Cell::class);
+
+        return $cellMock;
     }
 
     /**
diff --git a/tests/Spout/Common/Entity/Style/BorderTest.php b/tests/Spout/Common/Entity/Style/BorderTest.php
index fa1e7ef42..bb89af756 100644
--- a/tests/Spout/Common/Entity/Style/BorderTest.php
+++ b/tests/Spout/Common/Entity/Style/BorderTest.php
@@ -113,7 +113,7 @@ public function testAnyCombinationOfAllowedBorderPartsParams()
                     $border->addPart($borderPart);
                     $this->assertCount(1, $border->getParts());
 
-                    /** @var $part BorderPart */
+                    /** @var BorderPart $part */
                     $part = $border->getParts()[$allowedName];
 
                     $this->assertEquals($allowedStyle, $part->getStyle());
diff --git a/tests/Spout/Common/Entity/Style/ColorTest.php b/tests/Spout/Common/Entity/Style/ColorTest.php
index 3d27a1c5e..deee80311 100644
--- a/tests/Spout/Common/Entity/Style/ColorTest.php
+++ b/tests/Spout/Common/Entity/Style/ColorTest.php
@@ -11,7 +11,7 @@
 class ColorTest extends TestCase
 {
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestRGB()
     {
@@ -54,7 +54,7 @@ public function testRGB($red, $green, $blue, $expectedColor)
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestRGBAInvalidColorComponents()
     {
diff --git a/tests/Spout/Common/Helper/CellTypeHelperTest.php b/tests/Spout/Common/Helper/CellTypeHelperTest.php
index b5a47503e..701a406df 100644
--- a/tests/Spout/Common/Helper/CellTypeHelperTest.php
+++ b/tests/Spout/Common/Helper/CellTypeHelperTest.php
@@ -10,7 +10,7 @@
 class CellTypeHelperTest extends TestCase
 {
     /**
-     * @return array
+     * @return void
      */
     public function testIsEmpty()
     {
@@ -27,7 +27,7 @@ public function testIsEmpty()
     }
 
     /**
-     * @return array
+     * @return void
      */
     public function testIsNonEmptyString()
     {
@@ -44,7 +44,7 @@ public function testIsNonEmptyString()
     }
 
     /**
-     * @return array
+     * @return void
      */
     public function testIsNumeric()
     {
@@ -66,7 +66,7 @@ public function testIsNumeric()
     }
 
     /**
-     * @return array
+     * @return void
      */
     public function testIsBoolean()
     {
diff --git a/tests/Spout/Common/Helper/EncodingHelperTest.php b/tests/Spout/Common/Helper/EncodingHelperTest.php
index 8d3c158b3..9bb0e832f 100644
--- a/tests/Spout/Common/Helper/EncodingHelperTest.php
+++ b/tests/Spout/Common/Helper/EncodingHelperTest.php
@@ -4,6 +4,7 @@
 
 use Box\Spout\Common\Exception\EncodingConversionException;
 use Box\Spout\TestUsingResource;
+use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
 
 /**
@@ -14,7 +15,7 @@ class EncodingHelperTest extends TestCase
     use TestUsingResource;
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestGetBytesOffsetToSkipBOM()
     {
@@ -38,6 +39,7 @@ public function dataProviderForTestGetBytesOffsetToSkipBOM()
     public function testGetBytesOffsetToSkipBOM($fileName, $encoding, $expectedBytesOffset)
     {
         $resourcePath = $this->getResourcePath($fileName);
+        /** @var resource $filePointer */
         $filePointer = fopen($resourcePath, 'r');
 
         $encodingHelper = new EncodingHelper(new GlobalFunctionsHelper());
@@ -47,7 +49,7 @@ public function testGetBytesOffsetToSkipBOM($fileName, $encoding, $expectedBytes
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForIconvOrMbstringUsage()
     {
@@ -73,7 +75,7 @@ public function testAttemptConversionToUTF8ShouldThrowIfConversionFailed($should
         $helperStub->method('iconv')->willReturn(false);
         $helperStub->method('mb_convert_encoding')->willReturn(false);
 
-        /** @var EncodingHelper $encodingHelperStub */
+        /** @var EncodingHelper&MockObject $encodingHelperStub */
         $encodingHelperStub = $this->getMockBuilder('\Box\Spout\Common\Helper\EncodingHelper')
                         ->setConstructorArgs([$helperStub])
                         ->setMethods(['canUseIconv', 'canUseMbString'])
@@ -91,7 +93,7 @@ public function testAttemptConversionToUTF8ShouldThrowIfConversionNotSupported()
     {
         $this->expectException(EncodingConversionException::class);
 
-        /** @var EncodingHelper $encodingHelperStub */
+        /** @var EncodingHelper&MockObject $encodingHelperStub */
         $encodingHelperStub = $this->getMockBuilder('\Box\Spout\Common\Helper\EncodingHelper')
                         ->disableOriginalConstructor()
                         ->setMethods(['canUseIconv', 'canUseMbString'])
@@ -110,7 +112,7 @@ public function testAttemptConversionToUTF8ShouldThrowIfConversionNotSupported()
      */
     public function testAttemptConversionToUTF8ShouldReturnReencodedString($shouldUseIconv)
     {
-        /** @var EncodingHelper $encodingHelperStub */
+        /** @var EncodingHelper&MockObject $encodingHelperStub */
         $encodingHelperStub = $this->getMockBuilder('\Box\Spout\Common\Helper\EncodingHelper')
                         ->setConstructorArgs([new GlobalFunctionsHelper()])
                         ->setMethods(['canUseIconv', 'canUseMbString'])
@@ -118,6 +120,7 @@ public function testAttemptConversionToUTF8ShouldReturnReencodedString($shouldUs
         $encodingHelperStub->method('canUseIconv')->willReturn($shouldUseIconv);
         $encodingHelperStub->method('canUseMbString')->willReturn(true);
 
+        /** @var string $encodedString */
         $encodedString = iconv(EncodingHelper::ENCODING_UTF8, EncodingHelper::ENCODING_UTF16_LE, 'input');
         $decodedString = $encodingHelperStub->attemptConversionToUTF8($encodedString, EncodingHelper::ENCODING_UTF16_LE);
 
@@ -129,7 +132,7 @@ public function testAttemptConversionToUTF8ShouldReturnReencodedString($shouldUs
      */
     public function testAttemptConversionToUTF8ShouldBeNoopWhenTargetIsUTF8()
     {
-        /** @var EncodingHelper $encodingHelperStub */
+        /** @var EncodingHelper&MockObject $encodingHelperStub */
         $encodingHelperStub = $this->getMockBuilder('\Box\Spout\Common\Helper\EncodingHelper')
                         ->disableOriginalConstructor()
                         ->setMethods(['canUseIconv'])
@@ -156,7 +159,7 @@ public function testAttemptConversionFromUTF8ShouldThrowIfConversionFailed($shou
         $helperStub->method('iconv')->willReturn(false);
         $helperStub->method('mb_convert_encoding')->willReturn(false);
 
-        /** @var EncodingHelper $encodingHelperStub */
+        /** @var EncodingHelper&MockObject $encodingHelperStub */
         $encodingHelperStub = $this->getMockBuilder('\Box\Spout\Common\Helper\EncodingHelper')
                         ->setConstructorArgs([$helperStub])
                         ->setMethods(['canUseIconv', 'canUseMbString'])
@@ -174,7 +177,7 @@ public function testAttemptConversionFromUTF8ShouldThrowIfConversionNotSupported
     {
         $this->expectException(EncodingConversionException::class);
 
-        /** @var EncodingHelper $encodingHelperStub */
+        /** @var EncodingHelper&MockObject $encodingHelperStub */
         $encodingHelperStub = $this->getMockBuilder('\Box\Spout\Common\Helper\EncodingHelper')
                         ->disableOriginalConstructor()
                         ->setMethods(['canUseIconv', 'canUseMbString'])
@@ -193,7 +196,7 @@ public function testAttemptConversionFromUTF8ShouldThrowIfConversionNotSupported
      */
     public function testAttemptConversionFromUTF8ShouldReturnReencodedString($shouldUseIconv)
     {
-        /** @var EncodingHelper $encodingHelperStub */
+        /** @var EncodingHelper&MockObject $encodingHelperStub */
         $encodingHelperStub = $this->getMockBuilder('\Box\Spout\Common\Helper\EncodingHelper')
                         ->setConstructorArgs([new GlobalFunctionsHelper()])
                         ->setMethods(['canUseIconv', 'canUseMbString'])
@@ -212,7 +215,7 @@ public function testAttemptConversionFromUTF8ShouldReturnReencodedString($should
      */
     public function testAttemptConversionFromUTF8ShouldBeNoopWhenTargetIsUTF8()
     {
-        /** @var EncodingHelper $encodingHelperStub */
+        /** @var EncodingHelper&MockObject $encodingHelperStub */
         $encodingHelperStub = $this->getMockBuilder('\Box\Spout\Common\Helper\EncodingHelper')
                         ->disableOriginalConstructor()
                         ->setMethods(['canUseIconv'])
diff --git a/tests/Spout/Common/Helper/Escaper/ODSTest.php b/tests/Spout/Common/Helper/Escaper/ODSTest.php
index 858eb6882..c88a09464 100644
--- a/tests/Spout/Common/Helper/Escaper/ODSTest.php
+++ b/tests/Spout/Common/Helper/Escaper/ODSTest.php
@@ -11,7 +11,7 @@
 class ODSTest extends TestCase
 {
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestEscape()
     {
diff --git a/tests/Spout/Common/Helper/Escaper/XLSXTest.php b/tests/Spout/Common/Helper/Escaper/XLSXTest.php
index 3efe0e716..aeb8afdd2 100644
--- a/tests/Spout/Common/Helper/Escaper/XLSXTest.php
+++ b/tests/Spout/Common/Helper/Escaper/XLSXTest.php
@@ -11,7 +11,7 @@
 class XLSXTest extends TestCase
 {
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestEscape()
     {
@@ -46,7 +46,7 @@ public function testEscape($stringToEscape, $expectedEscapedString)
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestUnescape()
     {
diff --git a/tests/Spout/Common/Helper/FileSystemHelperTest.php b/tests/Spout/Common/Helper/FileSystemHelperTest.php
index 7eb018365..0eb314e9e 100644
--- a/tests/Spout/Common/Helper/FileSystemHelperTest.php
+++ b/tests/Spout/Common/Helper/FileSystemHelperTest.php
@@ -10,7 +10,7 @@
  */
 class FileSystemHelperTest extends TestCase
 {
-    /** @var \Box\Spout\Writer\XLSX\Helper\FileSystemHelper */
+    /** @var FileSystemHelper */
     protected $fileSystemHelper;
 
     /**
diff --git a/tests/Spout/Common/Manager/OptionsManagerTest.php b/tests/Spout/Common/Manager/OptionsManagerTest.php
index d5fe5d8e1..32fa1e571 100644
--- a/tests/Spout/Common/Manager/OptionsManagerTest.php
+++ b/tests/Spout/Common/Manager/OptionsManagerTest.php
@@ -17,7 +17,10 @@ class OptionsManagerTest extends TestCase
     protected function setUp() : void
     {
         $this->optionsManager = new class() extends OptionsManagerAbstract {
-            protected function getSupportedOptions()
+            /**
+             * @return string[]
+             */
+            protected function getSupportedOptions() : array
             {
                 return [
                     'foo',
diff --git a/tests/Spout/Reader/CSV/ReaderTest.php b/tests/Spout/Reader/CSV/ReaderTest.php
index 684b04542..9d88f0b94 100644
--- a/tests/Spout/Reader/CSV/ReaderTest.php
+++ b/tests/Spout/Reader/CSV/ReaderTest.php
@@ -6,11 +6,13 @@
 use Box\Spout\Common\Exception\IOException;
 use Box\Spout\Common\Helper\EncodingHelper;
 use Box\Spout\Common\Helper\GlobalFunctionsHelper;
+use Box\Spout\Common\Manager\OptionsManagerInterface;
 use Box\Spout\Reader\CSV\Creator\InternalEntityFactory;
 use Box\Spout\Reader\CSV\Manager\OptionsManager;
 use Box\Spout\Reader\Exception\ReaderNotOpenedException;
 use Box\Spout\Reader\ReaderInterface;
 use Box\Spout\TestUsingResource;
+use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
 
 /**
@@ -47,7 +49,7 @@ public function testOpenShouldThrowExceptionIfFileNotReadable()
     {
         $this->expectException(IOException::class);
 
-        /** @var \Box\Spout\Common\Helper\GlobalFunctionsHelper|\PHPUnit_Framework_MockObject_MockObject $helperStub */
+        /** @var \Box\Spout\Common\Helper\GlobalFunctionsHelper&MockObject $helperStub */
         $helperStub = $this->getMockBuilder('\Box\Spout\Common\Helper\GlobalFunctionsHelper')
                         ->setMethods(['is_readable'])
                         ->getMock();
@@ -66,7 +68,7 @@ public function testOpenShouldThrowExceptionIfCannotOpenFile()
     {
         $this->expectException(IOException::class);
 
-        /** @var \Box\Spout\Common\Helper\GlobalFunctionsHelper|\PHPUnit_Framework_MockObject_MockObject $helperStub */
+        /** @var \Box\Spout\Common\Helper\GlobalFunctionsHelper&MockObject $helperStub */
         $helperStub = $this->getMockBuilder('\Box\Spout\Common\Helper\GlobalFunctionsHelper')
                         ->setMethods(['fopen'])
                         ->getMock();
@@ -159,7 +161,7 @@ public function testReadShouldReturnEmptyLinesIfShouldPreserveEmptyRowsSet()
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestReadShouldReadEmptyFile()
     {
@@ -241,7 +243,7 @@ public function testReadShouldNotTruncateLineBreak()
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestReadShouldSkipBom()
     {
@@ -274,7 +276,7 @@ public function testReadShouldSkipBom($fileName, $fileEncoding)
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestReadShouldSupportNonUTF8FilesWithoutBOMs()
     {
@@ -302,7 +304,7 @@ public function testReadShouldSupportNonUTF8FilesWithoutBOMs($fileName, $fileEnc
         $allRows = [];
         $resourcePath = $this->getResourcePath($fileName);
 
-        /** @var \Box\Spout\Common\Helper\GlobalFunctionsHelper|\PHPUnit_Framework_MockObject_MockObject $helperStub */
+        /** @var \Box\Spout\Common\Helper\GlobalFunctionsHelper&MockObject $helperStub */
         $helperStub = $this->getMockBuilder('\Box\Spout\Common\Helper\GlobalFunctionsHelper')
                         ->setMethods(['function_exists'])
                         ->getMock();
@@ -475,8 +477,8 @@ public function testReadWithUnsupportedCustomStreamWrapper()
     }
 
     /**
-     * @param \Box\Spout\Common\Helper\GlobalFunctionsHelper|null $optionsManager
-     * @param \Box\Spout\Common\Manager\OptionsManagerInterface|null $globalFunctionsHelper
+     * @param OptionsManagerInterface|null $optionsManager
+     * @param GlobalFunctionsHelper|null $globalFunctionsHelper
      * @return ReaderInterface
      */
     private function createCSVReader($optionsManager = null, $globalFunctionsHelper = null)
@@ -494,7 +496,7 @@ private function createCSVReader($optionsManager = null, $globalFunctionsHelper
      * @param string $fieldEnclosure
      * @param string $encoding
      * @param bool $shouldPreserveEmptyRows
-     * @return array All the read rows the given file
+     * @return array<array> All the read rows the given file
      */
     private function getAllRowsForFile(
         $fileName,
diff --git a/tests/Spout/Reader/CSV/SheetTest.php b/tests/Spout/Reader/CSV/SheetTest.php
index d910d3216..82ae593a3 100644
--- a/tests/Spout/Reader/CSV/SheetTest.php
+++ b/tests/Spout/Reader/CSV/SheetTest.php
@@ -3,6 +3,7 @@
 namespace Box\Spout\Reader\CSV;
 
 use Box\Spout\Reader\Common\Creator\ReaderEntityFactory;
+use Box\Spout\Reader\SheetInterface;
 use Box\Spout\TestUsingResource;
 use PHPUnit\Framework\TestCase;
 
@@ -27,7 +28,7 @@ public function testReaderShouldReturnCorrectSheetInfos()
 
     /**
      * @param string $fileName
-     * @return Sheet
+     * @return SheetInterface
      */
     private function openFileAndReturnSheet($fileName)
     {
diff --git a/tests/Spout/Reader/CSV/SpoutTestStream.php b/tests/Spout/Reader/CSV/SpoutTestStream.php
index d66e8117c..77823c769 100644
--- a/tests/Spout/Reader/CSV/SpoutTestStream.php
+++ b/tests/Spout/Reader/CSV/SpoutTestStream.php
@@ -23,13 +23,16 @@ class SpoutTestStream
     /**
      * @param string $path
      * @param int $flag
-     * @return array
+     * @return array<mixed>
      */
     public function url_stat($path, $flag)
     {
         $filePath = $this->getFilePathFromStreamPath($path);
 
-        return stat($filePath);
+        /** @var array<mixed> $stat */
+        $stat = stat($filePath);
+
+        return $stat;
     }
 
     /**
@@ -56,7 +59,11 @@ public function stream_open($path, $mode, $options, &$opened_path)
 
         // the path is like "spout://csv_name" so the actual file name correspond the name of the host.
         $filePath = $this->getFilePathFromStreamPath($path);
-        $this->fileHandle = fopen($filePath, $mode);
+
+        /** @var resource $fileHandle */
+        $fileHandle = fopen($filePath, $mode);
+
+        $this->fileHandle = $fileHandle;
 
         return true;
     }
@@ -69,7 +76,10 @@ public function stream_read($numBytes)
     {
         $this->position += $numBytes;
 
-        return fread($this->fileHandle, $numBytes);
+        /** @var string $read */
+        $read = fread($this->fileHandle, $numBytes);
+
+        return $read;
     }
 
     /**
diff --git a/tests/Spout/Reader/Common/Manager/RowManagerTest.php b/tests/Spout/Reader/Common/Manager/RowManagerTest.php
index b67684830..2bb5fc802 100644
--- a/tests/Spout/Reader/Common/Manager/RowManagerTest.php
+++ b/tests/Spout/Reader/Common/Manager/RowManagerTest.php
@@ -15,7 +15,7 @@
 class RowManagerTest extends TestCase
 {
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestFillMissingIndexesWithEmptyCells()
     {
@@ -34,7 +34,7 @@ public function dataProviderForTestFillMissingIndexesWithEmptyCells()
      * @param Cell[]|null $rowCells
      * @param Cell[] $expectedFilledCells
      */
-    public function testFillMissingIndexesWithEmptyCells($rowCells, $expectedFilledCells)
+    public function testFillMissingIndexesWithEmptyCells($rowCells, $expectedFilledCells) : void
     {
         $rowManager = $this->createRowManager();
 
@@ -48,7 +48,7 @@ public function testFillMissingIndexesWithEmptyCells($rowCells, $expectedFilledC
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestIsEmptyRow()
     {
@@ -64,7 +64,7 @@ public function dataProviderForTestIsEmptyRow()
     /**
      * @dataProvider dataProviderForTestIsEmptyRow
      *
-     * @param array $cells
+     * @param array<Cell> $cells
      * @param bool $expectedIsEmpty
      * @return void
      */
@@ -81,10 +81,13 @@ public function testIsEmptyRow(array $cells, $expectedIsEmpty)
      */
     private function createRowManager()
     {
-        $entityFactory = new InternalEntityFactory(
-            $this->createMock(ManagerFactory::class),
-            $this->createMock(HelperFactory::class)
-        );
+        /** @var ManagerFactory $managerFactory */
+        $managerFactory = $this->createMock(ManagerFactory::class);
+
+        /** @var HelperFactory $helperFactory */
+        $helperFactory = $this->createMock(HelperFactory::class);
+
+        $entityFactory = new InternalEntityFactory($managerFactory, $helperFactory);
 
         return new RowManager($entityFactory);
     }
diff --git a/tests/Spout/Reader/ODS/ReaderTest.php b/tests/Spout/Reader/ODS/ReaderTest.php
index cd30ae54b..3bdf70dbf 100644
--- a/tests/Spout/Reader/ODS/ReaderTest.php
+++ b/tests/Spout/Reader/ODS/ReaderTest.php
@@ -16,7 +16,7 @@ class ReaderTest extends TestCase
     use TestUsingResource;
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestReadShouldThrowException()
     {
@@ -41,7 +41,7 @@ public function testReadShouldThrowException($filePath)
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestReadForAllWorksheets()
     {
@@ -109,7 +109,7 @@ public function testReadShouldSupportNumberColumnsRepeated()
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestReadWithFilesGeneratedByExternalSoftwares()
     {
@@ -538,7 +538,7 @@ public function testReaderShouldReadInlineFontFormattingAsText()
      * @param string $fileName
      * @param bool $shouldFormatDates
      * @param bool $shouldPreserveEmptyRows
-     * @return array All the read rows the given file
+     * @return array<array> All the read rows the given file
      */
     private function getAllRowsForFile($fileName, $shouldFormatDates = false, $shouldPreserveEmptyRows = false)
     {
diff --git a/tests/Spout/Reader/ODS/SheetTest.php b/tests/Spout/Reader/ODS/SheetTest.php
index 530e6c74a..cfd95c197 100644
--- a/tests/Spout/Reader/ODS/SheetTest.php
+++ b/tests/Spout/Reader/ODS/SheetTest.php
@@ -3,6 +3,7 @@
 namespace Box\Spout\Reader\ODS;
 
 use Box\Spout\Reader\Common\Creator\ReaderEntityFactory;
+use Box\Spout\Reader\SheetInterface;
 use Box\Spout\TestUsingResource;
 use PHPUnit\Framework\TestCase;
 
@@ -55,7 +56,7 @@ public function testReaderShouldReturnCorrectSheetVisibility()
 
     /**
      * @param string $fileName
-     * @return Sheet[]
+     * @return SheetInterface[]
      */
     private function openFileAndReturnSheets($fileName)
     {
diff --git a/tests/Spout/Reader/Wrapper/XMLReaderTest.php b/tests/Spout/Reader/Wrapper/XMLReaderTest.php
index 61aa7e7f1..0c6e4afcf 100644
--- a/tests/Spout/Reader/Wrapper/XMLReaderTest.php
+++ b/tests/Spout/Reader/Wrapper/XMLReaderTest.php
@@ -102,7 +102,7 @@ public function testNextShouldThrowExceptionOnError()
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestFileExistsWithinZip()
     {
@@ -134,10 +134,11 @@ public function testFileExistsWithinZip($innerFilePath, $expectedResult)
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestGetRealPathURIForFileInZip()
     {
+        /** @var string $tempFolder */
         $tempFolder = realpath(sys_get_temp_dir());
         $tempFolderName = basename($tempFolder);
         $expectedRealPathURI = 'zip://' . $tempFolder . '/test.xlsx#test.xml';
@@ -174,7 +175,7 @@ public function testGetRealPathURIForFileInZip($tempFolder, $zipFilePath, $fileI
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestIsPositionedOnStartingAndEndingNode()
     {
diff --git a/tests/Spout/Reader/XLSX/Helper/CellHelperTest.php b/tests/Spout/Reader/XLSX/Helper/CellHelperTest.php
index 87a65a669..9bdf7c9d9 100644
--- a/tests/Spout/Reader/XLSX/Helper/CellHelperTest.php
+++ b/tests/Spout/Reader/XLSX/Helper/CellHelperTest.php
@@ -11,7 +11,7 @@
 class CellHelperTest extends TestCase
 {
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestGetColumnIndexFromCellIndex()
     {
diff --git a/tests/Spout/Reader/XLSX/Helper/CellValueFormatterTest.php b/tests/Spout/Reader/XLSX/Helper/CellValueFormatterTest.php
index f981a63f9..1e044d43f 100644
--- a/tests/Spout/Reader/XLSX/Helper/CellValueFormatterTest.php
+++ b/tests/Spout/Reader/XLSX/Helper/CellValueFormatterTest.php
@@ -4,7 +4,9 @@
 
 use Box\Spout\Common\Helper\Escaper;
 use Box\Spout\Reader\Exception\InvalidValueException;
+use Box\Spout\Reader\XLSX\Manager\SharedStringsManager;
 use Box\Spout\Reader\XLSX\Manager\StyleManager;
+use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
 
 /**
@@ -13,7 +15,7 @@
 class CellValueFormatterTest extends TestCase
 {
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestExcelDate()
     {
@@ -72,6 +74,7 @@ public function testExcelDate($shouldUse1904Dates, $nodeValue, $expectedDateAsSt
             ->with(0)
             ->will($this->returnValue((object) ['nodeValue' => $nodeValue]));
 
+        /** @var \DOMElement&MockObject $nodeMock */
         $nodeMock = $this->createMock(\DOMElement::class);
 
         $nodeMock
@@ -88,7 +91,7 @@ public function testExcelDate($shouldUse1904Dates, $nodeValue, $expectedDateAsSt
             ->with(CellValueFormatter::XML_NODE_VALUE)
             ->will($this->returnValue($nodeListMock));
 
-        /** @var StyleManager|\PHPUnit_Framework_MockObject_MockObject $styleManagerMock */
+        /** @var StyleManager&MockObject $styleManagerMock */
         $styleManagerMock = $this->createMock(StyleManager::class);
 
         $styleManagerMock
@@ -97,7 +100,10 @@ public function testExcelDate($shouldUse1904Dates, $nodeValue, $expectedDateAsSt
             ->with(123)
             ->will($this->returnValue(true));
 
-        $formatter = new CellValueFormatter(null, $styleManagerMock, false, $shouldUse1904Dates, new Escaper\XLSX());
+        /** @var SharedStringsManager $sharedStringManager */
+        $sharedStringManager = $this->createMock(SharedStringsManager::class);
+
+        $formatter = new CellValueFormatter($sharedStringManager, $styleManagerMock, false, $shouldUse1904Dates, new Escaper\XLSX());
 
         try {
             $result = $formatter->extractAndFormatNodeValue($nodeMock);
@@ -106,7 +112,9 @@ public function testExcelDate($shouldUse1904Dates, $nodeValue, $expectedDateAsSt
                 $this->fail('An exception should have been thrown');
             } else {
                 $this->assertInstanceOf(\DateTime::class, $result);
-                $this->assertSame($expectedDateAsString, $result->format('Y-m-d H:i:s'));
+                /** @var \DateTime $datetime */
+                $datetime = $result;
+                $this->assertSame($expectedDateAsString, $datetime->format('Y-m-d H:i:s'));
             }
         } catch (InvalidValueException $exception) {
             // do nothing
@@ -114,7 +122,7 @@ public function testExcelDate($shouldUse1904Dates, $nodeValue, $expectedDateAsSt
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestFormatNumericCellValueWithNumbers()
     {
@@ -149,14 +157,17 @@ public function dataProviderForTestFormatNumericCellValueWithNumbers()
      */
     public function testFormatNumericCellValueWithNumbers($value, $expectedFormattedValue, $expectedType)
     {
-        /** @var StyleManager|\PHPUnit_Framework_MockObject_MockObject $styleManagerMock */
+        /** @var StyleManager&MockObject $styleManagerMock */
         $styleManagerMock = $this->createMock(StyleManager::class);
         $styleManagerMock
             ->expects($this->once())
             ->method('shouldFormatNumericValueAsDate')
             ->will($this->returnValue(false));
 
-        $formatter = new CellValueFormatter(null, $styleManagerMock, false, false, new Escaper\XLSX());
+        /** @var SharedStringsManager $sharedStringManager */
+        $sharedStringManager = $this->createMock(SharedStringsManager::class);
+
+        $formatter = new CellValueFormatter($sharedStringManager, $styleManagerMock, false, false, new Escaper\XLSX());
         $formattedValue = \ReflectionHelper::callMethodOnObject($formatter, 'formatNumericCellValue', $value, 0);
 
         $this->assertEquals($expectedFormattedValue, $formattedValue);
@@ -164,7 +175,7 @@ public function testFormatNumericCellValueWithNumbers($value, $expectedFormatted
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestFormatStringCellValue()
     {
@@ -203,7 +214,13 @@ public function testFormatInlineStringCellValue($value, $expectedFormattedValue)
             ->with(CellValueFormatter::XML_NODE_INLINE_STRING_VALUE)
             ->willReturn($nodeListMock);
 
-        $formatter = new CellValueFormatter(null, null, false, false, new Escaper\XLSX());
+        /** @var SharedStringsManager $sharedStringManager */
+        $sharedStringManager = $this->createMock(SharedStringsManager::class);
+
+        /** @var StyleManager $styleManager */
+        $styleManager = $this->createMock(StyleManager::class);
+
+        $formatter = new CellValueFormatter($sharedStringManager, $styleManager, false, false, new Escaper\XLSX());
         $formattedValue = \ReflectionHelper::callMethodOnObject($formatter, 'formatInlineStringCellValue', $nodeMock);
 
         $this->assertEquals($expectedFormattedValue, $formattedValue);
diff --git a/tests/Spout/Reader/XLSX/Helper/DateFormatHelperTest.php b/tests/Spout/Reader/XLSX/Helper/DateFormatHelperTest.php
index b2aea227a..92403cb8a 100644
--- a/tests/Spout/Reader/XLSX/Helper/DateFormatHelperTest.php
+++ b/tests/Spout/Reader/XLSX/Helper/DateFormatHelperTest.php
@@ -10,7 +10,7 @@
 class DateFormatHelperTest extends TestCase
 {
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestToPHPDateFormat()
     {
diff --git a/tests/Spout/Reader/XLSX/Manager/SharedStringsCaching/CachingStrategyFactoryTest.php b/tests/Spout/Reader/XLSX/Manager/SharedStringsCaching/CachingStrategyFactoryTest.php
index 807daf78a..01cccc0ba 100644
--- a/tests/Spout/Reader/XLSX/Manager/SharedStringsCaching/CachingStrategyFactoryTest.php
+++ b/tests/Spout/Reader/XLSX/Manager/SharedStringsCaching/CachingStrategyFactoryTest.php
@@ -3,6 +3,7 @@
 namespace Box\Spout\Reader\XLSX\Manager\SharedStringsCaching;
 
 use Box\Spout\Reader\XLSX\Creator\HelperFactory;
+use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
 
 /**
@@ -11,7 +12,7 @@
 class CachingStrategyFactoryTest extends TestCase
 {
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestCreateBestCachingStrategy()
     {
@@ -36,7 +37,7 @@ public function dataProviderForTestCreateBestCachingStrategy()
      */
     public function testCreateBestCachingStrategy($sharedStringsUniqueCount, $memoryLimitInKB, $expectedStrategyClassName)
     {
-        /** @var CachingStrategyFactory|\PHPUnit_Framework_MockObject_MockObject $factoryStub */
+        /** @var CachingStrategyFactory&MockObject $factoryStub */
         $factoryStub = $this
             ->getMockBuilder('\Box\Spout\Reader\XLSX\Manager\SharedStringsCaching\CachingStrategyFactory')
             ->disableOriginalConstructor()
@@ -46,7 +47,7 @@ public function testCreateBestCachingStrategy($sharedStringsUniqueCount, $memory
         $factoryStub->method('getMemoryLimitInKB')->willReturn($memoryLimitInKB);
 
         $tempFolder = sys_get_temp_dir();
-        $helperFactory = new HelperFactory($factoryStub);
+        $helperFactory = new HelperFactory();
         $strategy = $factoryStub->createBestCachingStrategy($sharedStringsUniqueCount, $tempFolder, $helperFactory);
 
         $fullExpectedStrategyClassName = 'Box\Spout\Reader\XLSX\Manager\SharedStringsCaching\\' . $expectedStrategyClassName;
@@ -56,7 +57,7 @@ public function testCreateBestCachingStrategy($sharedStringsUniqueCount, $memory
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestGetMemoryLimitInKB()
     {
@@ -84,7 +85,7 @@ public function dataProviderForTestGetMemoryLimitInKB()
      */
     public function testGetMemoryLimitInKB($memoryLimitFormatted, $expectedMemoryLimitInKB)
     {
-        /** @var CachingStrategyFactory|\PHPUnit_Framework_MockObject_MockObject $factoryStub */
+        /** @var CachingStrategyFactory&MockObject $factoryStub */
         $factoryStub = $this
             ->getMockBuilder('\Box\Spout\Reader\XLSX\Manager\SharedStringsCaching\CachingStrategyFactory')
             ->disableOriginalConstructor()
diff --git a/tests/Spout/Reader/XLSX/Manager/SharedStringsManagerTest.php b/tests/Spout/Reader/XLSX/Manager/SharedStringsManagerTest.php
index f344b3541..727490440 100644
--- a/tests/Spout/Reader/XLSX/Manager/SharedStringsManagerTest.php
+++ b/tests/Spout/Reader/XLSX/Manager/SharedStringsManagerTest.php
@@ -19,7 +19,7 @@ class SharedStringsManagerTest extends TestCase
 {
     use TestUsingResource;
 
-    /** @var SharedStringsManager */
+    /** @var SharedStringsManager|null */
     private $sharedStringsManager;
 
     /**
@@ -144,6 +144,7 @@ public function testGetStringAtIndexShouldNotDoubleDecodeHTMLEntities()
     public function testGetStringAtIndexWithFileBasedStrategy()
     {
         // force the file-based strategy by setting no memory limit
+        /** @var string $originalMemoryLimit */
         $originalMemoryLimit = ini_get('memory_limit');
         ini_set('memory_limit', '-1');
 
diff --git a/tests/Spout/Reader/XLSX/Manager/StyleManagerTest.php b/tests/Spout/Reader/XLSX/Manager/StyleManagerTest.php
index 794f4bc3e..7e35c5051 100644
--- a/tests/Spout/Reader/XLSX/Manager/StyleManagerTest.php
+++ b/tests/Spout/Reader/XLSX/Manager/StyleManagerTest.php
@@ -3,6 +3,7 @@
 namespace Box\Spout\Reader\XLSX\Manager;
 
 use Box\Spout\Reader\XLSX\Creator\InternalEntityFactory;
+use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
 
 /**
@@ -11,8 +12,8 @@
 class StyleManagerTest extends TestCase
 {
     /**
-     * @param array $styleAttributes
-     * @param array $customNumberFormats
+     * @param array<mixed> $styleAttributes
+     * @param array<mixed> $customNumberFormats
      * @return StyleManager
      */
     private function getStyleManagerMock($styleAttributes = [], $customNumberFormats = [])
@@ -21,7 +22,7 @@ private function getStyleManagerMock($styleAttributes = [], $customNumberFormats
         $workbookRelationshipsManager = $this->createMock(WorkbookRelationshipsManager::class);
         $workbookRelationshipsManager->method('hasStylesXMLFile')->willReturn(true);
 
-        /** @var StyleManager $styleManager */
+        /** @var StyleManager&MockObject $styleManager */
         $styleManager = $this->getMockBuilder('\Box\Spout\Reader\XLSX\Manager\StyleManager')
                              ->setConstructorArgs(['/path/to/file.xlsx', $workbookRelationshipsManager, $entityFactory])
                              ->setMethods(['getCustomNumberFormats', 'getStylesAttributes'])
@@ -132,7 +133,7 @@ public function testShouldFormatNumericValueAsDateWhenCustomNumberFormatNotFound
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForCustomDateFormats()
     {
diff --git a/tests/Spout/Reader/XLSX/ReaderPerfTest.php b/tests/Spout/Reader/XLSX/ReaderPerfTest.php
index 3cb97725a..bc3ff6c3f 100644
--- a/tests/Spout/Reader/XLSX/ReaderPerfTest.php
+++ b/tests/Spout/Reader/XLSX/ReaderPerfTest.php
@@ -15,7 +15,7 @@ class ReaderPerfTest extends TestCase
     use TestUsingResource;
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestPerfWhenReading300kRowsXLSX()
     {
diff --git a/tests/Spout/Reader/XLSX/ReaderTest.php b/tests/Spout/Reader/XLSX/ReaderTest.php
index 5b155b48a..8cda9de3d 100644
--- a/tests/Spout/Reader/XLSX/ReaderTest.php
+++ b/tests/Spout/Reader/XLSX/ReaderTest.php
@@ -15,7 +15,7 @@ class ReaderTest extends TestCase
     use TestUsingResource;
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestReadShouldThrowException()
     {
@@ -42,7 +42,7 @@ public function testReadShouldThrowException($filePath)
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestReadForAllWorksheets()
     {
@@ -748,7 +748,7 @@ public function testReaderShouldSupportStrictOOXML()
      * @param string $fileName
      * @param bool $shouldFormatDates
      * @param bool $shouldPreserveEmptyRows
-     * @return array All the read rows the given file
+     * @return array<array> All the read rows the given file
      */
     private function getAllRowsForFile($fileName, $shouldFormatDates = false, $shouldPreserveEmptyRows = false)
     {
diff --git a/tests/Spout/Reader/XLSX/SheetTest.php b/tests/Spout/Reader/XLSX/SheetTest.php
index a5389421b..25b16c0c3 100644
--- a/tests/Spout/Reader/XLSX/SheetTest.php
+++ b/tests/Spout/Reader/XLSX/SheetTest.php
@@ -3,6 +3,7 @@
 namespace Box\Spout\Reader\XLSX;
 
 use Box\Spout\Reader\Common\Creator\ReaderEntityFactory;
+use Box\Spout\Reader\SheetInterface;
 use Box\Spout\TestUsingResource;
 use PHPUnit\Framework\TestCase;
 
@@ -43,7 +44,7 @@ public function testReaderShouldReturnCorrectSheetVisibility()
 
     /**
      * @param string $fileName
-     * @return Sheet[]
+     * @return SheetInterface[]
      */
     private function openFileAndReturnSheets($fileName)
     {
diff --git a/tests/Spout/ReflectionHelper.php b/tests/Spout/ReflectionHelper.php
index c078f3bd0..4796afc4a 100644
--- a/tests/Spout/ReflectionHelper.php
+++ b/tests/Spout/ReflectionHelper.php
@@ -5,6 +5,9 @@
  */
 class ReflectionHelper
 {
+    /**
+     * @var array<class-string, array>
+     */
     private static $privateVarsToReset = [];
 
     /**
@@ -27,7 +30,7 @@ public static function reset()
      * Get the value of a static private or public class property.
      * Used to test internals of class without having to make the property public
      *
-     * @param string $class
+     * @param class-string $class
      * @param string $valueName
      * @return mixed|null
      */
@@ -48,7 +51,7 @@ public static function getStaticValue($class, $valueName)
      * Set the value of a static private or public class property.
      * Used to test internals of class without having to make the property public
      *
-     * @param string $class
+     * @param class-string $class
      * @param string $valueName
      * @param mixed|null $value
      * @param bool $saveOriginalValue
@@ -62,7 +65,7 @@ public static function setStaticValue($class, $valueName, $value, $saveOriginalV
 
         // to prevent side-effects in later tests, we need to remember the original value and reset it on tear down
         // @NOTE: we need to check isset in case the original value was null or array()
-        if ($saveOriginalValue && (!isset(self::$privateVarsToReset[$class]) || !isset(self::$privateVarsToReset[$class][$name]))) {
+        if ($saveOriginalValue && (!isset(self::$privateVarsToReset[$class]))) {
             self::$privateVarsToReset[$class][$valueName] = $reflectionProperty->getValue();
         }
         $reflectionProperty->setValue($value);
@@ -95,7 +98,6 @@ public static function getValueOnObject($object, $valueName)
      *
      * @param object $object
      * @param string $methodName
-     * @param *mixed|null $params
      *
      * @return mixed|null
      */
diff --git a/tests/Spout/TestUsingResource.php b/tests/Spout/TestUsingResource.php
index 551f86b61..d4c96f7fe 100644
--- a/tests/Spout/TestUsingResource.php
+++ b/tests/Spout/TestUsingResource.php
@@ -92,7 +92,13 @@ protected function createUnwritableFolderIfNeeded()
      */
     protected function getTempFolderPath()
     {
-        return realpath($this->tempFolderPath);
+        $path = realpath($this->tempFolderPath);
+
+        if ($path === false) {
+            throw new \RuntimeException(sprintf("Realpath of '%s' failed.", $path));
+        }
+
+        return $path;
     }
 
     /**
diff --git a/tests/Spout/Writer/CSV/WriterTest.php b/tests/Spout/Writer/CSV/WriterTest.php
index deb3f22b6..405208ffc 100644
--- a/tests/Spout/Writer/CSV/WriterTest.php
+++ b/tests/Spout/Writer/CSV/WriterTest.php
@@ -69,6 +69,7 @@ public function testAddRowsShouldThrowExceptionIfRowsAreNotArrayOfArrays()
         $this->expectException(InvalidArgumentException::class);
 
         $writer = WriterEntityFactory::createCSVWriter();
+        /* @phpstan-ignore-next-line */
         $writer->addRows([['csv--11', 'csv--12']]);
         $writer->close();
     }
@@ -176,7 +177,7 @@ public function testWriteShouldSupportCustomFieldEnclosure()
         $this->assertEquals('#This is, a comma#,csv--12,csv--13', $writtenContent, 'The fields should be enclosed with #');
     }
 
-    public function testWriteShouldSupportedEscapedCharacters()
+    public function testWriteShouldSupportedEscapedCharacters() : void
     {
         $allRows = $this->createRowsFromValues([
             ['"csv--11"', 'csv--12\\', 'csv--13\\\\', 'csv--14\\\\\\'],
@@ -193,7 +194,7 @@ public function testWriteShouldSupportedEscapedCharacters()
      * @param string $fieldDelimiter
      * @param string $fieldEnclosure
      * @param bool   $shouldAddBOM
-     * @return string|null
+     * @return string
      */
     private function writeToCsvFileAndReturnWrittenContent($allRows, $fileName, $fieldDelimiter = ',', $fieldEnclosure = '"', $shouldAddBOM = true)
     {
@@ -209,7 +210,10 @@ private function writeToCsvFileAndReturnWrittenContent($allRows, $fileName, $fie
         $writer->addRows($allRows);
         $writer->close();
 
-        return file_get_contents($resourcePath);
+        /** @var string $content */
+        $content = file_get_contents($resourcePath);
+
+        return $content;
     }
 
     /**
diff --git a/tests/Spout/Writer/Common/Entity/SheetTest.php b/tests/Spout/Writer/Common/Entity/SheetTest.php
index e0cf8eeb2..2326cb791 100644
--- a/tests/Spout/Writer/Common/Entity/SheetTest.php
+++ b/tests/Spout/Writer/Common/Entity/SheetTest.php
@@ -25,7 +25,7 @@ public function setUp() : void
 
     /**
      * @param int $sheetIndex
-     * @param int $associatedWorkbookId
+     * @param string $associatedWorkbookId
      * @return Sheet
      */
     private function createSheet($sheetIndex, $associatedWorkbookId)
@@ -57,7 +57,7 @@ public function testSetSheetNameShouldCreateSheetWithCustomName()
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForInvalidSheetNames()
     {
diff --git a/tests/Spout/Writer/Common/Helper/CellHelperTest.php b/tests/Spout/Writer/Common/Helper/CellHelperTest.php
index 3ab9a795a..33815d28d 100644
--- a/tests/Spout/Writer/Common/Helper/CellHelperTest.php
+++ b/tests/Spout/Writer/Common/Helper/CellHelperTest.php
@@ -10,7 +10,7 @@
 class CellHelperTest extends TestCase
 {
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestGetColumnLettersFromColumnIndex()
     {
diff --git a/tests/Spout/Writer/Common/Manager/RowManagerTest.php b/tests/Spout/Writer/Common/Manager/RowManagerTest.php
index 91ae37805..75b7553da 100644
--- a/tests/Spout/Writer/Common/Manager/RowManagerTest.php
+++ b/tests/Spout/Writer/Common/Manager/RowManagerTest.php
@@ -10,7 +10,7 @@
 class RowManagerTest extends TestCase
 {
     /**
-     * @return array
+     * @return array<mixed>
      */
     public function dataProviderForTestIsEmptyRow()
     {
@@ -26,7 +26,7 @@ public function dataProviderForTestIsEmptyRow()
     /**
      * @dataProvider dataProviderForTestIsEmptyRow
      *
-     * @param array $cells
+     * @param array<Cell> $cells
      * @param bool $expectedIsEmpty
      * @return void
      */
diff --git a/tests/Spout/Writer/ODS/SheetTest.php b/tests/Spout/Writer/ODS/SheetTest.php
index 3bd219366..64b99b178 100644
--- a/tests/Spout/Writer/ODS/SheetTest.php
+++ b/tests/Spout/Writer/ODS/SheetTest.php
@@ -87,6 +87,7 @@ public function testSetSheetVisibilityShouldCreateSheetHidden()
 
         $resourcePath = $this->getGeneratedResourcePath($fileName);
         $pathToContentFile = $resourcePath . '#content.xml';
+        /** @var string $xmlContents */
         $xmlContents = file_get_contents('zip://' . $pathToContentFile);
 
         $this->assertStringContainsString(' table:display="false"', $xmlContents, 'The sheet visibility should have been changed to "hidden"');
@@ -95,7 +96,7 @@ public function testSetSheetVisibilityShouldCreateSheetHidden()
     /**
      * @param string $fileName
      * @param string $sheetName
-     * @return Sheet
+     * @return void
      */
     private function writeDataAndReturnSheetWithCustomName($fileName, $sheetName)
     {
@@ -162,6 +163,7 @@ private function assertSheetNameEquals($expectedName, $fileName, $message = '')
     {
         $resourcePath = $this->getGeneratedResourcePath($fileName);
         $pathToWorkbookFile = $resourcePath . '#content.xml';
+        /** @var string $xmlContents */
         $xmlContents = file_get_contents('zip://' . $pathToWorkbookFile);
 
         $this->assertStringContainsString("table:name=\"$expectedName\"", $xmlContents, $message);
diff --git a/tests/Spout/Writer/ODS/WriterTest.php b/tests/Spout/Writer/ODS/WriterTest.php
index a9aaa0e39..6ad0288c2 100644
--- a/tests/Spout/Writer/ODS/WriterTest.php
+++ b/tests/Spout/Writer/ODS/WriterTest.php
@@ -284,19 +284,28 @@ public function testAddRowShouldSupportMultipleTypesOfData()
      */
     public function testAddRowShouldSupportFloatValuesInDifferentLocale()
     {
-        $previousLocale = \setlocale(LC_ALL, 0);
+        /** @var string[] $previousLocale */
+        $previousLocale = \setlocale(LC_ALL, '0');
 
         try {
             // Pick a supported locale whose decimal point is a comma.
             // Installed locales differ from one system to another, so we can't pick
             // a given locale.
             $supportedLocales = explode("\n", shell_exec('locale -a'));
+
+            $commaDecimalPointSupported = false;
             foreach ($supportedLocales as $supportedLocale) {
                 \setlocale(LC_ALL, $supportedLocale);
                 if (\localeconv()['decimal_point'] === ',') {
+                    $commaDecimalPointSupported = true;
                     break;
                 }
             }
+
+            if ($commaDecimalPointSupported === false) {
+                $this->markTestSkipped('System has no local which support comma decimal point');
+            }
+
             $this->assertEquals(',', \localeconv()['decimal_point']);
 
             $fileName = 'test_add_row_should_support_float_values_in_different_locale.xlsx';
@@ -306,8 +315,8 @@ public function testAddRowShouldSupportFloatValuesInDifferentLocale()
 
             $this->writeToODSFile($dataRows, $fileName);
 
-            $this->assertValueWasNotWrittenToSheet($fileName, 1, "1234,5");
-            $this->assertValueWasWrittenToSheet($fileName, 1, "1234.5");
+            $this->assertValueWasNotWrittenToSheet($fileName, 1, '1234,5');
+            $this->assertValueWasWrittenToSheet($fileName, 1, '1234.5');
         } finally {
             // reset locale
             \setlocale(LC_ALL, $previousLocale);
@@ -315,7 +324,7 @@ public function testAddRowShouldSupportFloatValuesInDifferentLocale()
     }
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestAddRowShouldUseNumberColumnsRepeatedForRepeatedValues()
     {
@@ -331,7 +340,7 @@ public function dataProviderForTestAddRowShouldUseNumberColumnsRepeatedForRepeat
     /**
      * @dataProvider dataProviderForTestAddRowShouldUseNumberColumnsRepeatedForRepeatedValues
      *
-     * @param array $dataRow
+     * @param array<mixed> $dataRow
      * @param int $expectedNumTableCells
      * @param int $expectedNumColumnsRepeated
      * @return void
@@ -341,6 +350,7 @@ public function testAddRowShouldUseNumberColumnsRepeatedForRepeatedValues($dataR
         $fileName = 'test_add_row_should_use_number_columns_repeated.ods';
         $this->writeToODSFile($this->createRowsFromValues([$dataRow]), $fileName);
 
+        /** @var \DOMElement $sheetXmlNode */
         $sheetXmlNode = $this->getSheetXmlNode($fileName, 1);
         $tableCellNodes = $sheetXmlNode->getElementsByTagName('table-cell');
 
@@ -580,6 +590,7 @@ private function assertValueWasWritten($fileName, $value, $message = '')
     {
         $resourcePath = $this->getGeneratedResourcePath($fileName);
         $pathToContentFile = $resourcePath . '#content.xml';
+        /** @var string $xmlContents */
         $xmlContents = file_get_contents('zip://' . $pathToContentFile);
 
         $this->assertStringContainsString($value, $xmlContents, $message);
@@ -624,7 +635,10 @@ private function getSheetXmlNode($fileName, $sheetIndex)
     {
         $xmlReader = $this->moveReaderToCorrectTableNode($fileName, $sheetIndex);
 
-        return $xmlReader->expand();
+        /** @var \DOMNode $node */
+        $node =  $xmlReader->expand();
+
+        return $node;
     }
 
     /**
diff --git a/tests/Spout/Writer/ODS/WriterWithStyleTest.php b/tests/Spout/Writer/ODS/WriterWithStyleTest.php
index 75177da2e..07e15e1a7 100644
--- a/tests/Spout/Writer/ODS/WriterWithStyleTest.php
+++ b/tests/Spout/Writer/ODS/WriterWithStyleTest.php
@@ -112,6 +112,7 @@ public function testAddRowShouldWriteDefaultStyleSettings()
 
         $this->writeToODSFile([$dataRow], $fileName);
 
+        /** @var \DOMElement $textPropertiesElement */
         $textPropertiesElement = $this->getXmlSectionFromStylesXmlFile($fileName, 'style:text-properties');
         $this->assertEquals(Style::DEFAULT_FONT_SIZE . 'pt', $textPropertiesElement->getAttribute('fo:font-size'));
         $this->assertEquals('#' . Style::DEFAULT_FONT_COLOR, $textPropertiesElement->getAttribute('fo:color'));
@@ -351,6 +352,7 @@ public function testSetDefaultRowStyle()
 
         $this->writeToODSFileWithDefaultStyle($dataRows, $fileName, $defaultStyle);
 
+        /** @var \DOMElement $textPropertiesElement */
         $textPropertiesElement = $this->getXmlSectionFromStylesXmlFile($fileName, 'style:text-properties');
         $this->assertEquals($defaultFontSize . 'pt', $textPropertiesElement->getAttribute('fo:font-size'));
     }
@@ -397,7 +399,7 @@ private function writeToODSFileWithDefaultStyle($allRows, $fileName, $defaultSty
 
     /**
      * @param string $fileName
-     * @return \DOMNode[]
+     * @return \DOMElement[]
      */
     private function getCellElementsFromContentXmlFile($fileName)
     {
@@ -410,7 +412,9 @@ private function getCellElementsFromContentXmlFile($fileName)
 
         while ($xmlReader->read()) {
             if ($xmlReader->isPositionedOnStartingNode('table:table-cell') && $xmlReader->getAttribute('office:value-type') !== null) {
-                $cellElements[] = $xmlReader->expand();
+                /** @var \DOMElement $element */
+                $element = $xmlReader->expand();
+                $cellElements[] = $element;
             }
         }
 
@@ -421,7 +425,7 @@ private function getCellElementsFromContentXmlFile($fileName)
 
     /**
      * @param string $fileName
-     * @return \DOMNode[]
+     * @return \DOMElement[]
      */
     private function getCellStyleElementsFromContentXmlFile($fileName)
     {
@@ -434,7 +438,10 @@ private function getCellStyleElementsFromContentXmlFile($fileName)
 
         while ($xmlReader->read()) {
             if ($xmlReader->isPositionedOnStartingNode('style:style') && $xmlReader->getAttribute('style:family') === 'table-cell') {
-                $cellStyleElements[] = $xmlReader->expand();
+                /** @var \DOMElement $element */
+                $element = $xmlReader->expand();
+
+                $cellStyleElements[] = $element;
             }
         }
 
@@ -456,18 +463,23 @@ private function getXmlSectionFromStylesXmlFile($fileName, $section)
         $xmlReader->openFileInZip($resourcePath, 'styles.xml');
         $xmlReader->readUntilNodeFound($section);
 
-        return $xmlReader->expand();
+        /** @var \DOMNode $node */
+        $node =  $xmlReader->expand();
+
+        return $node;
     }
 
     /**
      * @param string $expectedValue
-     * @param \DOMNode $parentElement
+     * @param \DOMElement $parentElement
      * @param string $childTagName
      * @param string $attributeName
      * @return void
      */
     private function assertFirstChildHasAttributeEquals($expectedValue, $parentElement, $childTagName, $attributeName)
     {
-        $this->assertEquals($expectedValue, $parentElement->getElementsByTagName($childTagName)->item(0)->getAttribute($attributeName));
+        /** @var \DOMElement $child */
+        $child = $parentElement->getElementsByTagName($childTagName)->item(0);
+        $this->assertEquals($expectedValue, $child->getAttribute($attributeName));
     }
 }
diff --git a/tests/Spout/Writer/RowCreationHelper.php b/tests/Spout/Writer/RowCreationHelper.php
index 0f3c6bbe0..db9f2374f 100644
--- a/tests/Spout/Writer/RowCreationHelper.php
+++ b/tests/Spout/Writer/RowCreationHelper.php
@@ -12,7 +12,7 @@
 trait RowCreationHelper
 {
     /**
-     * @param array $cellValues
+     * @param array<mixed> $cellValues
      * @return Row
      */
     protected function createRowFromValues(array $cellValues)
@@ -21,7 +21,7 @@ protected function createRowFromValues(array $cellValues)
     }
 
     /**
-     * @param array $cellValues
+     * @param array<mixed> $cellValues
      * @param Style|null $rowStyle
      * @return Row
      */
@@ -31,7 +31,7 @@ protected function createStyledRowFromValues(array $cellValues, Style $rowStyle
     }
 
     /**
-     * @param array $rowValues
+     * @param array<array> $rowValues
      * @return Row[]
      */
     protected function createRowsFromValues(array $rowValues)
@@ -40,7 +40,7 @@ protected function createRowsFromValues(array $rowValues)
     }
 
     /**
-     * @param array $rowValues
+     * @param array<array> $rowValues
      * @param Style|null $rowsStyle
      * @return Row[]
      */
diff --git a/tests/Spout/Writer/XLSX/Manager/Style/StyleManagerTest.php b/tests/Spout/Writer/XLSX/Manager/Style/StyleManagerTest.php
index 3bb0f42e2..76808ef6f 100644
--- a/tests/Spout/Writer/XLSX/Manager/Style/StyleManagerTest.php
+++ b/tests/Spout/Writer/XLSX/Manager/Style/StyleManagerTest.php
@@ -2,6 +2,7 @@
 
 namespace Box\Spout\Writer\XLSX\Manager\Style;
 
+use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
 
 /**
@@ -10,7 +11,7 @@
 class StyleManagerTest extends TestCase
 {
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestShouldApplyStyleOnEmptyCell()
     {
@@ -38,6 +39,7 @@ public function dataProviderForTestShouldApplyStyleOnEmptyCell()
      */
     public function testShouldApplyStyleOnEmptyCell($fillId, $borderId, $expectedResult)
     {
+        /** @var \Box\Spout\Writer\Common\Manager\Style\StyleRegistry&MockObject $styleRegistryMock */
         $styleRegistryMock = $this->getMockBuilder(StyleRegistry::class)
                                 ->disableOriginalConstructor()
                                 ->setMethods(['getFillIdForStyleId', 'getBorderIdForStyleId'])
diff --git a/tests/Spout/Writer/XLSX/SheetTest.php b/tests/Spout/Writer/XLSX/SheetTest.php
index a2431619c..53a3fbeec 100644
--- a/tests/Spout/Writer/XLSX/SheetTest.php
+++ b/tests/Spout/Writer/XLSX/SheetTest.php
@@ -87,6 +87,7 @@ public function testSetSheetVisibilityShouldCreateSheetHidden()
 
         $resourcePath = $this->getGeneratedResourcePath($fileName);
         $pathToWorkbookFile = $resourcePath . '#xl/workbook.xml';
+        /** @var string $xmlContents */
         $xmlContents = file_get_contents('zip://' . $pathToWorkbookFile);
 
         $this->assertStringContainsString(' state="hidden"', $xmlContents, 'The sheet visibility should have been changed to "hidden"');
@@ -164,6 +165,7 @@ private function assertSheetNameEquals($expectedName, $fileName, $message = '')
     {
         $resourcePath = $this->getGeneratedResourcePath($fileName);
         $pathToWorkbookFile = $resourcePath . '#xl/workbook.xml';
+        /** @var string $xmlContents */
         $xmlContents = file_get_contents('zip://' . $pathToWorkbookFile);
 
         $this->assertStringContainsString("<sheet name=\"$expectedName\"", $xmlContents, $message);
diff --git a/tests/Spout/Writer/XLSX/WriterPerfTest.php b/tests/Spout/Writer/XLSX/WriterPerfTest.php
index 66ff33797..84f1899dd 100644
--- a/tests/Spout/Writer/XLSX/WriterPerfTest.php
+++ b/tests/Spout/Writer/XLSX/WriterPerfTest.php
@@ -15,7 +15,7 @@ class WriterPerfTest extends TestCase
     use TestUsingResource;
 
     /**
-     * @return array
+     * @return array<array>
      */
     public function dataProviderForTestPerfWhenWritingOneMillionRowsXLSX()
     {
@@ -102,7 +102,7 @@ private function getNumWrittenRowsUsingSharedStrings($resourcePath)
 
     /**
      * @param string $filePath
-     * @return string
+     * @return int
      */
     private function getLasRowNumberForFile($filePath)
     {
diff --git a/tests/Spout/Writer/XLSX/WriterTest.php b/tests/Spout/Writer/XLSX/WriterTest.php
index 26b379257..6126a896c 100644
--- a/tests/Spout/Writer/XLSX/WriterTest.php
+++ b/tests/Spout/Writer/XLSX/WriterTest.php
@@ -120,6 +120,7 @@ public function testAddRowShouldThrowExceptionIfUnsupportedDataTypePassedIn()
             [str_repeat('a', WorksheetManager::MAX_CHARACTERS_PER_CELL + 1)],
         ];
 
+        /* @phpstan-ignore-next-line */
         $this->writeToXLSXFile($dataRows, $fileName);
     }
 
@@ -398,7 +399,8 @@ public function testAddRowShouldSupportCellInError()
      */
     public function testAddRowShouldSupportFloatValuesInDifferentLocale()
     {
-        $previousLocale = \setlocale(LC_ALL, 0);
+        /** @var string[] $previousLocale */
+        $previousLocale = \setlocale(LC_ALL, '0');
         $valueToWrite = 1234.5; // needs to be defined before changing the locale as PHP8 would expect 1234,5
 
         try {
@@ -406,12 +408,19 @@ public function testAddRowShouldSupportFloatValuesInDifferentLocale()
             // Installed locales differ from one system to another, so we can't pick
             // a given locale.
             $supportedLocales = explode("\n", shell_exec('locale -a'));
+
+            $commaDecimalPointSupported = false;
             foreach ($supportedLocales as $supportedLocale) {
                 \setlocale(LC_ALL, $supportedLocale);
                 if (\localeconv()['decimal_point'] === ',') {
+                    $commaDecimalPointSupported = true;
                     break;
                 }
             }
+            if ($commaDecimalPointSupported === false) {
+                $this->markTestSkipped('System has no local which support comma decimal point');
+            }
+
             $this->assertEquals(',', \localeconv()['decimal_point']);
 
             $fileName = 'test_add_row_should_support_float_values_in_different_locale.xlsx';
@@ -421,8 +430,8 @@ public function testAddRowShouldSupportFloatValuesInDifferentLocale()
 
             $this->writeToXLSXFile($dataRows, $fileName, $shouldUseInlineStrings = false);
 
-            $this->assertInlineDataWasNotWrittenToSheet($fileName, 1, "1234,5");
-            $this->assertInlineDataWasWrittenToSheet($fileName, 1, "1234.5");
+            $this->assertInlineDataWasNotWrittenToSheet($fileName, 1, '1234,5');
+            $this->assertInlineDataWasWrittenToSheet($fileName, 1, '1234.5');
         } finally {
             // reset locale
             \setlocale(LC_ALL, $previousLocale);
@@ -642,6 +651,7 @@ private function assertInlineDataWasWrittenToSheet($fileName, $sheetIndex, $inli
     {
         $resourcePath = $this->getGeneratedResourcePath($fileName);
         $pathToSheetFile = $resourcePath . '#xl/worksheets/sheet' . $sheetIndex . '.xml';
+        /** @var string $xmlContents */
         $xmlContents = file_get_contents('zip://' . $pathToSheetFile);
 
         $this->assertStringContainsString((string) $inlineData, $xmlContents, $message);
@@ -658,6 +668,7 @@ private function assertInlineDataWasNotWrittenToSheet($fileName, $sheetIndex, $i
     {
         $resourcePath = $this->getGeneratedResourcePath($fileName);
         $pathToSheetFile = $resourcePath . '#xl/worksheets/sheet' . $sheetIndex . '.xml';
+        /** @var string $xmlContents */
         $xmlContents = file_get_contents('zip://' . $pathToSheetFile);
 
         $this->assertStringNotContainsString((string) $inlineData, $xmlContents, $message);
@@ -673,6 +684,7 @@ private function assertSharedStringWasWritten($fileName, $sharedString, $message
     {
         $resourcePath = $this->getGeneratedResourcePath($fileName);
         $pathToSharedStringsFile = $resourcePath . '#xl/sharedStrings.xml';
+        /** @var string $xmlContents */
         $xmlContents = file_get_contents('zip://' . $pathToSharedStringsFile);
 
         $this->assertStringContainsString($sharedString, $xmlContents, $message);
diff --git a/tests/Spout/Writer/XLSX/WriterWithStyleTest.php b/tests/Spout/Writer/XLSX/WriterWithStyleTest.php
index 2571df416..c174151c9 100644
--- a/tests/Spout/Writer/XLSX/WriterWithStyleTest.php
+++ b/tests/Spout/Writer/XLSX/WriterWithStyleTest.php
@@ -394,10 +394,16 @@ public function testReuseBackgroundColorSharedDefinition()
             '3 cell xfs present - a default one and two custom ones'
         );
 
-        $firstCustomId = $styleXfsElements->childNodes->item(1)->getAttribute('fillId');
+        /** @var \DOMElement $firstItem */
+        $firstItem = $styleXfsElements->childNodes->item(1);
+
+        $firstCustomId = $firstItem->getAttribute('fillId');
         $this->assertEquals(2, (int) $firstCustomId, 'The first custom fill id should have the index 2');
 
-        $secondCustomId = $styleXfsElements->childNodes->item(2)->getAttribute('fillId');
+        /** @var \DOMElement $secondItem */
+        $secondItem = $styleXfsElements->childNodes->item(2);
+
+        $secondCustomId = $secondItem->getAttribute('fillId');
         $this->assertEquals(2, (int) $secondCustomId, 'The second custom fill id should have the index 2');
     }
 
@@ -468,7 +474,6 @@ public function testBordersCorrectOrder()
             $borderParts = $borderNode->childNodes;
             $ordering = [];
 
-            /** @var \DOMText $part */
             foreach ($borderParts as $part) {
                 if ($part instanceof \DOMElement) {
                     $ordering[] = $part->nodeName;
@@ -617,6 +622,7 @@ private function getXmlSectionFromStylesXmlFile($fileName, $section)
         $xmlReader->openFileInZip($resourcePath, 'xl/styles.xml');
         $xmlReader->readUntilNodeFound($section);
 
+        /** @var \DOMElement $xmlSection */
         $xmlSection = $xmlReader->expand();
 
         $xmlReader->close();
@@ -626,7 +632,7 @@ private function getXmlSectionFromStylesXmlFile($fileName, $section)
 
     /**
      * @param string $fileName
-     * @return \DOMNode[]
+     * @return \DOMElement[]
      */
     private function getCellElementsFromSheetXmlFile($fileName)
     {
@@ -639,7 +645,9 @@ private function getCellElementsFromSheetXmlFile($fileName)
 
         while ($xmlReader->read()) {
             if ($xmlReader->isPositionedOnStartingNode('c')) {
-                $cellElements[] = $xmlReader->expand();
+                /** @var \DOMElement $element */
+                $element = $xmlReader->expand();
+                $cellElements[] = $element;
             }
         }
 
@@ -650,19 +658,21 @@ private function getCellElementsFromSheetXmlFile($fileName)
 
     /**
      * @param string $expectedValue
-     * @param \DOMNode $parentElement
+     * @param \DOMElement $parentElement
      * @param string $childTagName
      * @param string $attributeName
      * @return void
      */
     private function assertFirstChildHasAttributeEquals($expectedValue, $parentElement, $childTagName, $attributeName)
     {
-        $this->assertEquals($expectedValue, $parentElement->getElementsByTagName($childTagName)->item(0)->getAttribute($attributeName));
+        /** @var \DOMElement $child */
+        $child = $parentElement->getElementsByTagName($childTagName)->item(0);
+        $this->assertEquals($expectedValue, $child->getAttribute($attributeName));
     }
 
     /**
      * @param int $expectedNumber
-     * @param \DOMNode $parentElement
+     * @param \DOMElement $parentElement
      * @param string $message
      * @return void
      */
@@ -672,7 +682,7 @@ private function assertChildrenNumEquals($expectedNumber, $parentElement, $messa
     }
 
     /**
-     * @param \DOMNode $parentElement
+     * @param \DOMElement $parentElement
      * @param string $childTagName
      * @return void
      */