PHP Training Classes

These are the training classes I followed to prepare myself for the Zend certification. I hope you find them as useful as they were (and still are) for me 😃.

Installation

Dump autoloader:

composer dump-autoload

Usage

Subjects can be executed through the trainer PHP command. Below, each exercise will indicate its signature within the bash command example (if available).

php trainer {exercise-signature}

Subjects

Superglobals

Superglobals are variables that are available automatically to the script, with no need for global keyword usage. Superglobals are available in every scope.

Suberglobal	Content
$GLOBALS	An array of variables that exist in the global scope.
$_SERVER	An array of information about paths, headers, and other information relevant to the server environment.
$_GET	Variables sent in a GET request.
$_POST	Variables sent in a POST request.
$_FILES	An associative array of files that were uploaded as part of a POST request.
$_COOKIE	An associative array of variables passed to the current script via HTTP cookies.
$_SESSION	An associative array containing session variables available to the current script.
$_REQUEST	POST, GET, and COOKIE request variables.
$_ENV	An associative array of variables passed to the current script via the environment method.

The $_SERVER superglobal contains a lot of keys, and it is important to get familiar with its values.

▶️ Run exercise:

php trainer superglobals

Note: Unlike the HTML form methods like POST and GET, other non-HTML form methods such as PATCH, PUT, DELETE, etc. don't have their superglobal. To access them, you need to use the php://input stream.

Constants

Constants are similar to variables but are immutable. They have the same naming rules as variables, but by convention will have uppercase names.

define(name: 'PI', value: 3.142, case_insensitive: false);
defined('PI'); // true
echo PI; // 3.142

Note: Constants can only contain arrays or scalar values and not resources or objects.

Only the const keyword can be used to create a namespaced constant.

// Math.php
namespace Math;
const PI = 3.142;
define('EPSILON', 0.001);

// Test.php
namespace Test;
echo \Math\PI; // 3.142
echo \Math\EPSILON; // Fatal error!
echo EPSILON; // 0.001

Note: Assume Math.php is loaded when Test.php is executed.

▶️ Run exercise:

php trainer constants:namespaced

Magic Constants

Nine constants change depending on where they are used, these are called magic constants. For example, the value of __LINE__ depends on the line that it's used in the script. All these "magical" constants are resolved at compile-time, unlike regular constants, which are resolved at runtime. These special constants are case-insensitive and are as follows:

Constant	Content
__LINE__	The line number of the PHP executed script
__DIR__	The directory of the file. If used inside an include, the directory of the included file is returned. This is equivalent to dirname(__FILE__). This directory name does not have a trailing slash unless it is the root directory
__FILE__	The fully resolved (including symlinks) name and path of the file being executed
__CLASS__	The name of the class being executed
__METHOD__	The name of the class method being executed
__FUNCTION__	The name of the function being executed
__TRAIT__	The namespace and name of the trait that the code is running in
__NAMESPACE__	The current namespace

▶️ Run exercise:

php trainer constants:magic-constants

Predefined Constants

These constants are defined by the PHP core. This includes PHP, the Zend engine, and SAPI modules.

Constant	Type	Content
PHP_VERSION	string	The current PHP version as a string in "major.minor.release[extra]" notation.
PHP_MAJOR_VERSION	int	The current PHP "major" version as an integer (e.g., int(8) from version 8.0.7-extra).
PHP_MINOR_VERSION	int	The current PHP "minor" version as an integer (e.g., int(0) from version 8.0.7-extra).
PHP_RELEASE_VERSION	int	The current PHP "release" version as an integer (e.g., int(7) from version 8.0.7-extra).
PHP_VERSION_ID	int	The current PHP version as an integer, useful for version comparisons (e.g., int(80007) from version 8.0.7-extra).
PHP_EXTRA_VERSION	string	The current PHP "extra" version as a string (e.g., '-extra' from version 8.0.7-extra). Often used by distribution vendors to indicate a package version.
PHP_ZTS	int	N/A
PHP_DEBUG	int	N/A
PHP_MAXPATHLEN	int	The maximum length of filenames (including path) supported by this build of PHP.
PHP_OS	string	The operating system PHP was built for.
PHP_OS_FAMILY	string	The operating system family PHP was built for. One of Windows, BSD, Darwin, Solaris, Linux or Unknown.
PHP_SAPI	string	The Server API for this build of PHP. See also php_sapi_name().
PHP_EOL	string	The correct 'End Of Line' symbol for this platform.
PHP_INT_MAX	int	The largest integer supported in this build of PHP. Usually int(2147483647) in 32 bit systems and int(9223372036854775807) in 64 bit systems.
PHP_INT_MIN	int	The smallest integer supported in this build of PHP. Usually int(-2147483648) in 32 bit systems and int(-9223372036854775808) in 64 bit systems. Usually, PHP_INT_MIN === ~PHP_INT_MAX.
PHP_INT_SIZE	int	The size of an integer in bytes in this build of PHP.
PHP_FLOAT_DIG	int	Number of decimal digits that can be rounded into a float and back without precision loss.
PHP_FLOAT_EPSILON	float	Smallest representable positive number x, so that x + 1.0 != 1.0.
PHP_FLOAT_MIN	float	Smallest representable positive floating point number. If you need the smallest representable negative floating point number, use PHP_FLOAT_MAX.
PHP_FLOAT_MAX	float	Largest representable floating point number.
DEFAULT_INCLUDE_PATH	string	N/A
PEAR_INSTALL_DIR	string	N/A
PEAR_EXTENSION_DIR	string	N/A
PHP_EXTENSION_DIR	string	The default directory where to look for dynamically loadable extensions (unless overridden by extension_dir). Defaults to PHP_PREFIX (or PHP_PREFIX . "\\ext" on Windows).
PHP_PREFIX	string	The value --prefix was set to at configure. On Windows, it is the value --with-prefix was set to at configure.
PHP_BINDIR	string	The value --bindir was set to at configure. On Windows, it is the value --with-prefix was set to at configure.
PHP_BINARY	string	Specifies the PHP binary path during script execution.
PHP_MANDIR	string	Specifies where the manpages were installed into.
PHP_LIBDIR	string	N/A
PHP_DATADIR	string	N/A
PHP_SYSCONFDIR	string	N/A
PHP_LOCALSTATEDIR	string	N/A
PHP_CONFIG_FILE_PATH	string	N/A
PHP_CONFIG_FILE_SCAN_DIR	string	N/A
PHP_SHLIB_SUFFIX	string	The build-platform's shared library suffix, such as "so" (most Unixes) or "dll" (Windows).
PHP_FD_SETSIZE	string	The maximum number of file descriptors for select system calls.

Error Reporting Constants

Constant	Type	Description	Value
E_ERROR	int	Fatal run-time errors. These indicate errors that can not be recovered from, such as a memory allocation problem. The execution of the script is halted.	1
E_WARNING	int	Run-time warnings (non-fatal errors). Execution of the script is not halted.	2
E_PARSE	int	Compile-time parse errors. Parse errors should only be generated by the parser.	4
E_NOTICE	int	Run-time notices. Indicate that the script encountered something that could indicate an error, but could also happen in the normal course of running a script.	8
E_CORE_ERROR	int	Fatal errors that occur during PHP's initial startup. This is like an E_ERROR, except it is generated by the core of PHP.	16
E_CORE_WARNING	int	Warnings (non-fatal errors) that occur during PHP's initial startup. This is like an E_WARNING, except it is generated by the core of PHP.	32
E_COMPILE_ERROR	int	Fatal compile-time errors. This is like an E_ERROR, except it is generated by the Zend Scripting Engine.	64
E_COMPILE_WARNING	int	Compile-time warnings (non-fatal errors). This is like an E_WARNING, except it is generated by the Zend Scripting Engine.	128
E_USER_ERROR	int	User-generated error message. This is like an E_ERROR, except it is generated in PHP code by using the PHP function trigger_error().	256
E_USER_WARNING	int	User-generated warning message. This is like an E_WARNING, except it is generated in PHP code by using the PHP function trigger_error().	512
E_USER_NOTICE	int	User-generated notice message. This is like an E_NOTICE, except it is generated in PHP code by using the PHP function trigger_error().	1024
E_STRICT	int	Enable to have PHP suggest changes to your code which will ensure the best interoperability and forward compatibility of your code.	2048
E_RECOVERABLE_ERROR	int	Catchable fatal error. It indicates that a probably dangerous error occurred, but did not leave the Engine in an unstable state. If the error is not caught by a user-defined handle (see also set_error_handler()), the application aborts as it was an E_ERROR.	4096
E_DEPRECATED	int	Run-time notices. Enable this to receive warnings about code that will not work in future versions.	8192
E_USER_DEPRECATED	int	User-generated warning message. This is like an E_DEPRECATED, except it is generated in PHP code by using the PHP function trigger_error().	16384
E_ALL	int	All errors and warnings, as supported.	32767

The above values (either numerical or symbolic) are used to build up a bitmask that specifies which errors to report. You can use the bitwise operators to combine these values or mask out certain types of errors. Note that only |, ~, !, ^ and & will be understood within php.ini. See also error-reporting().

Operators

Logic Operators

PHP uses both symbol-based and word-based logic operators. The symbol-based operators are C-based.

Operator	Example	True When
and	$a and $b	Both $a and $b evaluate true
and	$a && $b	Both $a and $b evaluate true
or	$a or $b	Either $a or $b evaluate true
or	$a || $b	Either $a or $b evaluate true
xor	$a xor $b	One of (but not both) $a or $b is true
xor	$a ^ $b	One of (but not both) $a or $b is true
not	!$a	$a is not true (false)

It is better (a good practice actually) not to mix both forms (symbol/word-based) in the same comparison. Even though both serve the same purpose, they have different precedence. For that reason is safer to use the symbol-based form only.

$a = true;
$b = false;
$word = $a and $b; // $word = true
$symbol = $a && $b; // $symbol = false
assert($word === $symbol);

The above is due to and and or have lower precedence than =, so the comparison happens after the assignment. You can visualize like this:

($word = $a) and $b;

At this point $word has the value true because it is assigned, and the and $b part happens next but is practically being ignored because the result of the comparison is never stored in a variable.

Ouch 🤕.

▶️ Run exercise:

php trainer operator:logic-precedence

Bitwise Operators

Bitwise operators work on the bits of integers represented in binary form. Every integer has its representation in binary, for example:

Decimal	Binary
0	0
1	1
2	10
4	100
8	1000
16	10000
17	10001

In PHP you'd represent binaries by prepending 0b to the binary number, for example 16 would be 0b10000.

There are three standard logical bitwise operators.

Note: In the following examples, when we say "a bit set" it means "there is a 1 at that position". Oppositely, when we say "a bit is not set" it means "there is a 0 at that position".

& (AND)

The result will have a bit set if both of the bits of the operands were set.

It's easier to see how these operators behave in a table. Let's use 25 and 50 with the & (AND) operator:

Value/Operator	Bits in Each Position
50	1	1	0	0	1	0
25	0	1	1	0	0	1
Operation	1 & 0	1 & 1	0 & 1	0 & 0	1 & 0	0 & 1
Result	0	1	0	0	0	0

The result is 010000 (or 10000) which is 16 in decimal, therefore 50 & 25 is 16.

| (OR)

If one or both of the operands have a bit set then the result will have that bit set.

Value/Operator	Bits in Each Position
50	1	1	0	0	1	0
25	0	1	1	0	0	1
Operation	1 | 0	1 | 1	0 | 1	0 | 0	1 | 0	0 | 1
Result	1	1	1	0	1	1

The result is 111011 which is 59 in decimal, therefore 50 | 25 is 59.

^ (XOR)

If one and only one of the operands (not both) has the bit set then the result will have the bit set.

Value/Operator	Bits in Each Position
50	1	1	0	0	1	0
25	0	1	1	0	0	1
Operation	1 ^ 0	1 ^ 1	0 ^ 1	0 ^ 0	1 ^ 0	0 ^ 1
Result	1	0	1	0	1	1

The result is 101011 which is 43 in decimal, therefore 50 ^ 25 is 43.

Using any of these operators on a different variable type will cause PHP to cast the variable to integer before operating on it. Therefore, using them on non-numeric strings will cause comparisons to 0 (the reason why 'not a number' == 0 results in true, by the way).

~ (NOT)

The effect of this operator is to flip the bits in a value—if a bit is set it becomes unset, and if it were not set it becomes set.

Value/Operator	Bits in Each Position
50	1	1	0	0	1	0
~50	0	0	1	1	0	1

Bit Shifting

PHP also has operators to shift bits left and right. The effect of these operators is to shift the bit pattern of the value either left or right while inserting bits set to 0 in the newly created empty spaces.

Let's shift 50:

Value/Operator	Operation	Result	Result (Decimal)
50	None	0 1 1 0 0 1 0	50
50 >> 1	Shift right by 1	0 0 1 1 0 0 1	25
50 << 1	Shift left by 1	1 1 0 0 1 0 0	100

So, you're moving 110010 left and right.

When shifting left by n, you add n zeros to the right and don't remove any digits on the left (in the example below we add [] and leading zeros to the initial value to make it easier to see the movement), effectively multiplying the decimal value ×2n (or ×2 n times):

Bit Shift	Binary	Exponential	Multiplication	Result
50 << 0	000[110010]	50 × 20	50 × (1)	50
50 << 1	00[110010]0	50 × 21	50 × (2)	100
50 << 2	0[110010]00	50 × 22	50 × (2 × 2)	200
50 << 3	[110010]000	50 × 23	50 × (2 × 2 × 2)	400

When shifting right by n, you remove n digits on the right (in the example below we add [] and leading zeros to the values to make it easier to see the movement), effectively dividing the decimal value by 2n (or by 2 n times) while losing precision.

Bit Shift	Binary	Exponential	Multiplication	Result
50 >> 0	[110010]	50 ÷ 20	50 ÷ (1)	50
50 >> 1	0[11001	50 ÷ 21	50 ÷ (2)	25
50 >> 2	00[1100	50 ÷ 22	50 ÷ (2 × 2)	12
50 >> 3	000[110	50 ÷ 23	50 ÷ (2 × 2 × 2)	6

Tip: The base_convert() and decbin() functions are extremely useful. For example, to output the binary representation of the decimal number 50, you can use base_convert(50, 10, 2) or decbin(50).

Spaceship Operator (PHP ^7.0)

It returns -1, 0 or 1 when $a is respectively less than, equal to, or greater than $b. Comparisons are performed according to PHP's usual type comparison rules.

$a <=> $b;

Full example: src/Operators/Spaceship.php

▶️ Run exercise:

php trainer operator:spaceship

Null Coalesce Operator (PHP ^7.0)

It returns the value of $a if it's defined, otherwise, it will return the fallback.

echo $a ?? '❌ Variable not defined.';

Full example: src/Operators/NullCoalesce.php

▶️ Run exercise:

php trainer operator:null-coalesce

Nullsafe Operator (PHP ^8.0)

It runs a check if the value is defined in a chain-like syntax making the code cleaner for when a value in a chain might not be defined.

$personTwo->profile?->title;

Full example: src/Operators/Nullsafe.php

▶️ Run exercise:

php trainer operator:nullsafe

Casting Types

Boolean

PHP casts values to boolean false if they are:

• Integers (int) (also float) of value 0 (zero)
  • Which means negative values cast to true
• Strings (string) with empty value '' or with value '0' (zero)
• Arrays (array) with no elements
• Null (null) variables

String to Numbers

When casting strings to numeric types (such as int or float), PHP will default to 0 (zero) if the string does not start with a numeric value. If it does start with a numeric value, but then has a non-numeric value and later another (set of) numeric value(s), these will be ignored.

echo (int) '12 o clock'; // 12
echo (int) 'Half past 12'; // 0
echo (int) '12.30'; // 12
echo (int) '7.2e2 minutes after midnight'; // 720

Floats to Integers

Casting floating point values to integers truncates the value; no calculation occurs:

echo (int) 1234.99; // 1234
echo (int) -1234.99; // -1234

Floats to Integers in Array Keys

When using floats in array keys, PHP will do the same conversion logic as seen previously, replacing any previous value with the same result.

$array = [
    0   => 'zero',
    0.5 => 'half',
    1   => 'one',
    1.5 => 'one and a half'
];

[
    0 => 'half', // 0.5 casted to integer as 0, replaces 0.
    1 => 'one and a half', // 1.5 casted to integer as 1, replaces 1.
]

Cheat Sheets

Case Sensitiveness

PHP follows different rules across entities/members in terms of case sensitiveness. Here's a list of the most important ones:

Case Sensitive

• Constants
• Variables
• Array keys

define('PI', 3.1416);
const VERSION = 7;
$name = 'Harry PHPotter';
$complex = ['a' => 'HermioneJS'];

echo pi; // Use of undefined constant pi, assumes 'pi' as string.
echo version; // Use of undefined constant pi, assumes 'version' as string.
echo $Name; // Undefined variable: Name.
echo $complex['A']; // Undefined index: A.

Case Insensitive

• Namespaces
• Classes
• Methods
• Functions

namespace App\Middleware;

class Localize {
    public static function getLocale() {
        echo 'en_US';
    }
}

// In another file.
App\MiddleWare\Localize::GetLocale(); // en_US.

Strings

In functions that search in strings, the $haystack goes before the $needle. For example: strpos(), strstr(), strchr().

Embedding Strings

PHP can embed strings stored in variables by using the " (double quotes) and depending on the value access complexity, {} to separate it from the rest of the string.

$name = 'Vladimir';
$complex = ['alias' => 'Vlass'];

// My name is Vladimir, but you can call me Vlass.
echo "My name is $name, but you can call me {$complex['alias']}.";

If you were using single quotes ', then you wouldn't be embedding the variables, just showing them literally:

$name = 'Vladimir';
$complex = ['alias' => 'Vlass'];

// My name is $name, but you can call me {$complex["alias"]}.
echo 'My name is $name, but you can call me {$complex["alias"]}.';

In addition, HEREDOC is another way of embedding strings:

$name = 'Vladimir';
$complex = ['alias' => 'Vlass'];

// My name is Vladimir, but you can call me Vlass.
echo <<<HEREDOC
My name is $name, but you can call me {$complex['alias']}.
HEREDOC;

NOWDOC has the same effect as using single quotes:

$name = 'Vladimir';
$complex = ['alias' => 'Vlass'];

// My name is $name, but you can call me {$complex['alias']}.
echo <<<'NOWDOC'
My name is $name, but you can call me {$complex['alias']}.
NOWDOC;

Arrays

In functions that search in arrays, the $needle goes before the $haystack. For example: in_array(), array_search().

Array Functions

array_combine()

array_combine(array $keys, array $values): array

It creates an associative array using the elements of $keys as keys, and the elements of $values as values. Both parameters must have the same number of elements.

array_combine(['a', 'b'], [1, 2])

// Output.
[
    'a' => 1,
    'b' => 2,
]

array_merge()

array_merge(...array $arrays): array

It will append and/or replace any values from the previous array going from left to right. In other words, ((($array1, $array2), $array3), $array4)....

array_merge(
    [
        'a' => 1,
        'b' => 2,
        7   => 'good'
    ],
    [
        'b' => 3,
        'c' => 4,
        'd' => 5,
        7   => 'luck'
    ]
)

// Output
[
    'a' => 1,
    'b' => 3,      // Overridden, took right side.
    0   => 'good', // Renumbered
    'c' => 4,      // Appended
    'd' => 5,      // Appended
    1   => 'luck', // Renumbered + appended
]

The difference between array_merge() and the + operator is that the second will ignore values on the right hand if the key exists in the left hand, and unlike array_merge() it also includes numeric keys, and will not renumber them.

[
    'a' => 1,
    'b' => 2,
    7   => 'good'
] + [
    'b' => 3,
    'c' => 4,
    'd' => 5,
    7   => 'luck'
]

// Output
[
    'a' => 1,
    'b' => 2,      // Kept left side, right side ignored.
    7   => 'good', // Kept left side, right side ignored.
    'c' => 4,      // Appended
    'd' => 5,      // Appended
]

array_shift()

array_shift(array &$array): mixed

It will return the first element of the array and will remove it from the array. Numeric keys will be renumbered accordingly.

$array = [
    'one'   => '1st',
    2       => '2nd',
    'three' => '3rd'
];
$first = array_shift($array);

// $first
'1st'

// Array
[
    0       => '2nd',
    'three' => '3rd'
]

array_unshift()

array_unshift(array &$array, array ...$values): int

It will prepend the given $values at the beginning of the $array. The opposite of array_shift(). It also renumbers the numeric indexes.

The return value is the new number of elements in the $array.

$array = [
    2       => '2nd',
    'three' => '3rd'
];

array_unshift($array, '4th');

// $array
[
    0       => '4th', // The value we just inserted.
    1       => '2nd', // Renumbered.
    'three' => '3rd', // Nothing exciting here.
]

array_pop()

array_pop(array &$array): mixed

It will return the last element of the array and will remove it from the array. Numeric keys will be preserved.

$array = [
    'one'   => '1st',
    2       => '2nd',
    'three' => '3rd'
];
$last = array_pop($array);

// $last
'3rd'

// Array
[
    'one'   => '1st',
    2       => '2nd',
]

array_push()

array_push(array &$array, array ...$values): int

It will append the given $values at the end of the $array. The opposite of array_pop(). It also preserves the numeric indexes.

The return value is the new number of elements in the $array.

$array = [
    'one'   => '1st',
    2       => '2nd',
];

array_push($array, '3rd');

// $array
[
    'one' => '1sr',
    2     => '2nd',
    3     => '3rd', // Last numeric index + 1.
]

array_slice()

array_slice(
    array $array,
    int $offset,
    int|null $length = null,
    bool $preserve_keys = false
): array

It returns the $length items starting from $offset from $array leaving it untouched 😌.

$array = ['a' => 1, 'b' => 2, 42 => 'answer', 'c' => 3];
$slices = array_slice($array, 1, 2)

// $slices
[
    'b' => 2,
    0   => 'answer',
]

Note: The $preserve_keys applies for numeric keys only, as you can see we lost the 42, but we set it to true, we would preserve the 42.

array_splice()

array_splice(
    array $array,
    int $offset,
    int|null $length = null,
    mixed $replacement = []
): array

The return value of this function is the same as in array_slice(), but this one removes the slice from the $array and gives you the chance to replace it with $replacement.

Numeric keys will be renumbered nonetheless.

$array = ['a' => 1, 'b' => 2, 42 => 'answer', 'c' => 3];
$slices = array_splice($array, 1, 1);

// $slices
[
    'b' => 2,
]

// $array
[
    'a' => 1,
    // If we added a $replacement, that would be added here.
    0   => 'answer',
    'c' => 3,
]

Imagine it's like extracting a slice of 🍕.

Objects

Serialization

When serializing objects, the serialize() function will try to find the __serialize() and __sleep() methods.

public function __sleep(): array
{
    return [
        'name',
        'email',
        'phone',
    ];
}

This method must return an array containing the property names that should be included in the serialization.

public function __serialize(): array
{
    return [
        'name'  => $this->name,
        'email' => $this->email,
        'phone' => $this->phone,
    ];
}

This method must return an associative array containing the property names with their respective values that represent the serialized form of the object.

As you can see, __serialize() overrides the behavior of __sleep() because you can include as many keys as you want. Therefore, if both __serialize() and __sleep() are defined, only __serialize() will be called.

Trait Method Conflict

When multiple traits implement methods with the same name, you should pick which one to implement using the insteadof keyword or all of them with aliases using the as keyword:

trait CommandTrait
{
    public function run()
    {
        //
    }
}

trait PersonTrait
{
    public function run()
    {
        //
    }
}

class Person
{
    use CommandTrait;
    use PersonTrait {
        PersonTrait::run insteadof CommandTrait;
        CommandTrait::run as runCommand;
    }
}

(new Person())->run(); // PersonTrait::run()
(new Person())->runCommand(); // CommandTrait::run()

SPL (Standard PHP Library)

The Standard PHP Library (SPL) is a collection of interfaces and classes that are meant to solve common problems. It provides a set of standard data structures, iterators to traverse over objects, interfaces, standard Exceptions, a number of classes to work with files and autoloading functions.

ArrayObject

This class allows objects to work as arrays.

$fruits = [
    'P' => 'Android 9',
    'Q' => 'Android 10',
    'R' => 'Android 11',
    'S' => 'Android 12'
];

$fruitArrayObject = new ArrayObject($fruits, ArrayObject::ARRAY_AS_PROPS);
$fruitArrayObject->ksort();

foreach ($fruitArrayObject as $key => $val) {
    echo "The codeletter of $val is $key\n";
}

// This works because of ArrayObject::ARRAY_AS_PROPS, can also be set with ArrayObject::setFlags()
echo $fruitArrayObject->S; // Android 12

Generators

The purpose of generators is to mitigate the memory and time problems when accessing/manipulating big iterables. Instead of putting everything in memory at once, we delegate the control individually, per iteration.

$values = range(1, 1000000000);

foreach ($values as $value) {
    echo $value . PHP_EOL;
}

The above most likely will cause a memory exhausted error. Generators come to the rescue!

function generate(int $start, int $end) {
    for ($i = $start; $i <= $end; $i++) {
        yield $i;
    }
}

$values = generate(1, 1000000000);

foreach ($values as $value) {
    echo $value . PHP_EOL;
}

The script will run from 1 to 1,000,000,000, no memory exhausted!

Features

Grouped Imports (PHP ^7.0)

Instead of defining the entire classpath, we can group imports of a namespace into one curly brace group.

use Trainer\General\GroupedImports\Types\{
    Person,
    Animal,
    Environment\Water,
};

Full example: src/General/GroupedImports/GroupedImports.php.

▶️ Run exercise:

php trainer grouped-imports

Anonymous Classes (PHP ^7.0)

These classes are not files, but code blocks that define a nameless class just like anonymous functions. Use with caution! Suitable for cases where you want to change things on the fly, tests with mockery, debugging, etc.

someFunction(new class extends SomeClass implements SomeInterface  {
    public function someMethod(): void
    {
        //
    }
});

Full example: src/General/AnonymousClasses/AnonymousClasses.php.

▶️ Run exercise:

php trainer anonymous-classes

Matched Expressions (PHP ^8.0)

These are expressions that assign the proper value to $matched if the given $value matches, in a function + array-like syntax. Pretty much like a clean replacement for switch.

$matched = match ($value) {
    'Conversation'        => 'Started to conversation',
    'Comment'             => 'Added a new comment',
    'Reply', 'Subcomment' => 'Replied to conversation',
    'Attack'              => helperFunction(),
    default               => 'No action needed',
};

Full example: src/General/MatchedExpressions/MatchedExpressions.php.

▶️ Run exercise:

php trainer anonymous-classes

Constructor Property Promotion (PHP ^8.0)

This feature cleans up the constructor repetition by assigning the property visibility on the constructor parameter. Notice that this still behaves like normal properties when it comes to primitive vs. non-primitive initializations, hence, you can only auto-initialize a property with primitives, not expressions.

class Person
{
    public function __construct(protected string $name)
    {
    }
}

Full example: src/General/ConstructorPropertyPromotion/ConstructorPropertyPromotion.php.

▶️ Run exercise:

php trainer constructor-property-promotion

Dynamic Class Access (PHP ^8.0)

The ::class value of a class is also available in objects on a variable. Functionally equivalent to get_class().

$object = new SomeClass();
echo $object::class;

$object::class === get_class($object);

Full example: src/General/DynamicClassAccess/DynamicClassAccess.php.

▶️ Run exercise:

php trainer dynamic-class-access

Named Parameters (PHP ^8.0)

A function or class method with parameters can be invoked with those parameters in any order (hence, not the order they were in the function definition) as long as they include the name. Notice that this approach couples the function calls to the actual parameter names in the function definition.

$invoice = new Invoice(
    description: 'Web Development Services',
    chargeDate: new DateTime(),
    amount: 1000,
    paid: true
);

Full example: src/General/NamedParameters/NamedParameters.php.

▶️ Run exercise:

php trainer named-parameters

String Helpers (PHP ^8.0)

To determine if a string starts or ends with another string, or if a string contains another string.

$id = 'ch_1IzO7K2eZvKYlo2CIGjQqdcy';
str_starts_with($id, 'ch_'); // true.
str_ends_with($id, 'dcy'); // true.
str_contains($id, 'o4Fs'); // false.

Full example: src/General/StringHelpers/StringHelpers.php.

▶️ Run exercise:

php trainer string-helpers

Weak Maps (PHP ^8.0)

A map (or dictionary) that accepts objects as keys. Unlike SplObjectStorage, an object in a key of WeakMap does not contribute toward the object's reference count. That is, if at any point the only remaining reference to an object is the key of a WeakMap, the object will be garbage collected and removed from the WeakMap.

$object = new stdClass();
$store = new WeakMap();

$store[$object] = 'Anything'; // Now the object is in the map.

unset($object); // It's also removed from the map.

Full example: src/General/WeakMaps/WeakMaps.php.

▶️ Run exercise:

php trainer weak-maps

Union Types (PHP ^8.0)

When a function/method can handle multiple (but specific) types of arguments, we can type-hint those in the function definition. Notice that this kind of type hinting existed in PHP 7 with the ? before the type token, but it was just a union between the given type and null.

function cancel(string | DateTime $when)
{
    print_r($when);
}

// It works.
cancel('next week');
cancel(new DateTime());

// It won't.
cancel(new stdClass());

Full example: src/General/UnionTypes/UnionTypes.php.

▶️ Run exercise:

php trainer union-types

Mixed Pseudo Type (PHP ^8.0)

In terms of consistency, if you just like to type-hint everything, you can use the mixed keyword which its behavior is the same as not putting it. It indicates that the value for the parameter can be of any type.

function cancel(mixed $when) // function cancel($when) {}
{
    print_r($when);
}

// It works.
cancel('next week');
cancel(new DateTime());
cancel(new stdClass());
cancel(12);

Object-Oriented Principles

In Object-Oriented Programming (or OOP) we make use of classes to represent concepts that exist in an application. These classes might be interpreted as blueprints or templates that define the structure and behavior of these concepts.

To create a class, we first find the noun of what it represents. For example a comment, a person, a course, a video, achievement badge, etc. Those are represented like this (using the achievement badge as an example):

class AchievementBadge {
}

The pieces of data that make an achievement badge are properties of that concept, and are also called class properties in the OOP context.

class AchievementBadge {
    public $title;
    public $description;
    public $points;
}

At this point, we may say we have set the structure/blueprint of our concept. Now every time this class is instantiated (or in other words, creating objects of this class), each of those instances will have its own title, description, and points.

In addition, a class can have behavior. Let's say that our achievement badge can be awarded to a person; we can represent that through a class method:

class AchievementBadge {
    ...

    public function awardTo($person)
    {
        echo "$person has achieved $this->title";
    }
}

Ecapsulation

In these examples, properties and methods are prepended with the public keyword, this is the visibility, and public is the default (yet it is recommended to define it explicitly like in these examples). This is how you determine encapsulation.

Encapsulation allows a class to provide signals to the outside world that certain internals are private and shouldn't be accessed. So at its core, encapsulation is about communication.

But what it means?

• public: Makes the property or method publicly available to anyone accessing them, inside the same class, outside the class, or derived classes (classes that extend it).
• protected: Makes the property or method accessible only within the same class or its derived classes.
• private: Makes the property or method accessible only within the same class.

Inheritance

It allows an object to acquire (or inherit) the traits and behaviors of another object.

class CoffeeMaker
{
    public function brew()
    {
        echo 'Brewing coffee.';
    }
}

class SpecialtyCoffeeMake extends CoffeeMaker
{
    public function brewLatte()
    {
        echo 'Brewing latte';
    }
}

(new SpecialtyCoffeeMake())->brewLatte();
(new SpecialtyCoffeeMake())->brew(); // Inherits the ability to brew coffee.

Full example: src/OOP/Inheritance/Inheritance.php.

▶️ Run exercise:

php trainer oop:inheritance

Abstract Classes

Abstract classes provide templates or a base for any subclasses. A class is abstract when the abstract keyword is added before the class keyword on the class definition. Unlike interfaces, abstract classes can define properties and behavior.

Adding abstract to a class:

• Removes the ability to instantiate the class directly
• It can define abstract methods, which cannot be implemented directly as it needs specifics of the subclass, therefore the subclass must define its behavior

abstract class AchievementType
{
    public string $emoji = '🏆';

    public function getName(): string
    {
        return static::class;
    }

    public function getIconFilename(): string
    {
        return $this->getName() . '.png';
    }

    abstract public function setQualifier(string $user): string;
}

Full example: src/OOP/AbstractClasses/AbstractClasses.php.

▶️ Run exercise:

php trainer oop:abstract-classes

Interfaces

Interfaces are classes without behavior, they only have method signatures. They describe the terms for a particular contract, and any class that signs this contract must adhere to those terms. In other words, interfaces don't care about the specifics of the behavior, just that the subclasses define that behavior.

This reduces the chances of errors due to duck typing and handshakes (accepting any type as dependency hoping that it has a certain method).

interface NewsletterProvider
{
    public function subscribe(string $email): bool;
}

class SendGridNewsletterProvider implements NewsletterProvider
{
    public function subscribe(string $email): bool
    {
        // SendGrid logic.
    }
}

class CampaignMonitorNewsletterProvider implements NewsletterProvider
{
    public function subscribe(string $email): bool
    {
        // Campaign Monitor logic.
    }
}

class NewsletterController
{
    public function store(NewsletterProvider $provider, string $email) // We can use any class implementing NewsletterProvider
    {
        $provider->subscribe($email); // We know by contract it has this method.
    }
}

Full example: src/OOP/Interfaces/Interfaces.php.

Important: In the controller class of the full example, we implement a concept called Object Composition. Make sure you take a look at the constructor of that class to get more details 😉.

▶️ Run exercise:

php trainer oop:interfaces

Value Objects and Mutability

A value object is an object whose equality is determined by its data (or value) rather than any particular identity.

To illustrate this, imagine three five-dollar bills resting on a table. Does one bill have a unique identity compared to the other two? From our perspective, no. Five dollars is five dollars regardless of which bill you choose. However, compare this with two human beings who have the same first and last name. Are they identical, or does each person have a unique identity? Of course, in this case, the latter is the correct answer.

class Person
{
    protected string $firstName;
    protected string $lastName;

    public function __construct(string $firstName, string $lastName)
    {
        $this->firstName = $firstName;
        $this->lastName = $lastName;
    }
}

$personOne = new Person('John', 'Doe');
$personTwo = new Person('John', 'Doe');

The name attributes are often shared across a system because you might instantiate different Person objects and they have the same properties and might even have the same values, but are different objects.

Mutability (ability to change the internal state of an object) often opens the possibility to bypass any processes at the moment of setting the attribute of an object (like validation, events, etc.).

Mutable:

class Age
{
    public int $value;

    public function __construct(int $value)
    {
        if ($value < 0 || $value > 120) {
            throw new InvalidArgumentException('Invalid age');
        }

        $this->value = $value;
    }
}

$age = new Age(300); // Exception: Invalid age.
$age = new Age(43); // Valid age.
$age->value = 300; // Invalid age, yet the object has it assigned, validation bypassed.

Immutable:

class Age
{
    protected int $value;

    public function __construct(int $value)
    {
        if ($value < 0 || $value > 120) {
            throw new InvalidArgumentException('Invalid age');
        }

        $this->value = $value;
    }
}

$age = new Age(300); // Exception: Invalid age.
$age = new Age(43); // Valid age.
$age->value = 300; // Fatal error, access to protected property, unable to change the internal state.

Benefits:

• Avoids primitive obsession
• Helps with readability
• Helps with consistency (validations, formatting, etc.)
• By avoiding setters, we provide mutability (which tends to reduce bugs due to internal state changes)

Note: Be careful, do not map every primitive to a value object since it might unnecessary and increase complexity on maintainability.

Exceptions

These objects are thrown to indicate exceptional behavior. For example, when a business logic happens to fail or reach a sad path and a simple false return is not specific enough. Also, when an action can fail in multiple ways, and each way needs to be handled differently.

The objective is to have a clean and consistent way of handling failed actions. Exceptions bubble to parents (just like JavaScript events) until one of those parents handles it.

function add($a, $b) {
    if (! (is_numeric($a) || is_numeric($b))) {
        throw new InvalidArgumentException('The given values must be numeric.');
    }

    return $a + $b;
}

try {
    add(12, []);
} catch (InvalidArgumentException $e) {
    // Handle exceptional behavior.
}

Full example: src/OOP/Exceptions/Exceptions.php

▶️ Run exercise:

php trainer oop:exceptions

SOLID Principles

These are good practices that help the developer write code that is more scalable and maintainable.

Single Responsibility Principle

The S in SOLID. This principle dictates that each class should do just one thing.

$salesReport = new SalesReporter();

$formatter = new HtmlOutput();
echo $salesReport->between(new DateTime(), new DateTime(), $formatter);

$formatter = new PlainOutput();
echo $salesReport->between(new DateTime(), new DateTime(), $formatter);

Full example: src/Principles/SingleResponsibility/SingleResponsibility.php

▶️ Run exercise:

php trainer principle:single-responsibility

Open-Closed Principle

The O in SOLID. This principle dictates that classes must be closed for modification but open for extension.

$receipt = new Receipt(1000);
$checkout = new Checkout();

// Can accept any implementation of the appropriate interface.
$stripe = new StripePaymentGateway();
$checkout->pay($receipt, $stripe);

// This one works too!
$paypal = new PayPalPaymentGateway();
$checkout->pay($receipt, $paypal);

Full example: src/Principles/OpenClosed/OpenClosed.php

▶️ Run exercise:

php trainer principle:open-closed

Liskov Substitution Principle

The L in SOLID. This principle dictates that derived classes (implementation of an abstract class or interface) must be substitutable for their base class (anywhere where the abstraction is accepted). Essentially, any LSP-compliant class should have the same contract including return types.

$fileReporitory = new FileLessonRepository();
$databaseRepository = new DatabaseLessonRepository();

/**
 * We should be able to use either of them since both follow
 * the same contract with the same return types, and all.
 */
echo showLessons($fileReporitory) . PHP_EOL;
echo showLessons($databaseRepository) . PHP_EOL;

Full example: src/Principles/LiskovSubstitution/LiskovSubstitution.php

▶️ Run exercise:

php trainer principle:liskov-substitution

Interface Segregation

The I in SOLID. This principle dictates clients should not be forced to implement an interface they don't use. This can be interpreted and implemented thinking on the knowledge each class needs from others.

If a class TypeA only needs a little portion of data or behavior of a given object, that's when this principle suggests to segregate the implementation and only ask for an abstraction or interface InterfaceA, so any class implementing it can be used by TypeA since what the InterfaceA contract introduces is all that we need.

$captain = new Captain();

// Androids work.
$captain->manage(new AndroidWorker());

// Humans work, but also sleep.
$captain->manage(new HumanWorker());

Full example: src/Principles/InterfaceSegregation/InterfaceSegregation.php

▶️ Run exercise:

php trainer principle:interface-segregation

Dependency Inversion

The D in SOLID. This principle dictates that high-level (isn't concerned about details and specifics) modules should not depend on low-level (it is more concerned about details and specifics) modules. Instead, they should depend on abstractions, not concretions. The low-level module should also depend on abstractions.

$databaseConnection = new DatabaseConnection(); // Conforms to ConnectionInterface.

/**
 * The password reminder expects a ConnectionInterface, not the
 * DataBaseConnection concretion hence depends on an abstraction as
 * well, not a concretion.
 */
$passwordReminder = new PasswordReminder($databaseConnection);

Full example: src/Principles/DependencyInversion/DependencyInversion.php

▶️ Run exercise:

php trainer principle:dependency-inversion

Design Patterns

Decorator

When a class needs to interact with another class without breaking the OCP, we make use of decorators.

Decorators are classes in which constructors wrap another object who are instances of the same interface and add behavior to have this wrapped object into account.

$oilChange = new OilChangeDecorator(new BasicInspection());
$oilChange->getCost();

Full example: src/DesignPatterns/Decorator/Decorator.php

▶️ Run exercise:

php trainer pattern:decorator

Adapter

This pattern is essentially what it sounds like. When an object is not compatible with an implementation but is in the same context, we can use adapters.

Adapters are classes that wrap an object of an incompatible type and translate the equivalent methods to the implemented class.

$person = new Person(); // Can read books.
$book = new Book();
$eBook = new Kindle();

$person->read($book);
$person->read(new EReaderAdapter($eBook)); // Adapt eBook to Book.

Full example: src/DesignPatterns/Adapter/Adapter.php

▶️ Run exercise:

php trainer pattern:adapter

Template Method

This pattern is used to illustrate the steps of an algorithm in class methods, but implementations might differ in some steps.

abstract class Sub
{
    public function make()
    {
        return $this
            ->layBread()
            ->addLettuce()
            ->addPrimaryToppings()
            ->addSauces();
    }
}

Full example: src/DesignPatterns/TemplateMethod/TemplateMethod.php

▶️ Run exercise:

php trainer pattern:template-method

Strategy

This pattern is used to define a family of algorithms and make them interchangeable.

$logger = new Logger();

$logger->logMessage('I am a message', new FileLogger());
$logger->logMessage('I am a message', new DatabaseLogger());
$logger->logMessage('I am a message', new WebServiceLogger());

Full example: src/DesignPatterns/Strategy/Strategy.php

▶️ Run exercise:

php trainer pattern:strategy

Chain of Responsibility

This pattern is used to connect objects that can either handle or stop the execution of a request. This is done by storing a successor on each object, so in case one of the items in the chain did not result in a stopped execution, can instruct the next item to run its logic.

$lightsChecker = new LightsChecker();
$lockChecker = new LockChecker();
$alarmChecker = new AlarmChecker();

// Connect the chain.
$lightsChecker->setSuccessor($lockChecker);
$lockChecker->setSuccessor($alarmChecker);

// Now let's make the check "request".
$homeStatus = new HomeStatus(
    false, // lights
    true, // locked
    false // alarm
);

$lightsChecker->check($homeStatus);

Full example: src/DesignPatterns/ResponsibilityChain/ResponsibilityChain.php

▶️ Run exercise:

php trainer pattern:responsibility-chain

Observer

This pattern is used to observe the execution of actions in a class (subject), but this subject class doesn't need to be aware of anything of those observers. A list of observers subscribes (or observes) to a subject, and the subject notifies all of them once the triggering action occurs.

Pretty much like: Event happens -> Notify listeners.

// Auth is a subject.
$auth = new Auth();

// We attach observers to that subject.
$auth->attach([
    new LogObserver(),
    new EmailObserver(),
    new TraceObserver(),
]);

// And then we execute the action that notifies the obsevers.
$auth->notify();

Full example: src/DesignPatterns/Observer/Observer.php

▶️ Run exercise:

php trainer pattern:observer

Databases and SQL

PHP interacts with databases via drivers. It includes the MySQL driver called mysqli (the i at the end standing for improved), which is the replacement for mysql and introduces features such as prepared statements.

You can create your own driver by using PHP's PDO (PHP Data Objects) abstraction layer.

Data Types

Just like PHP variables, SQL has several data types.

Numeric Types

These are MySQL integer types. Other databases might handle them differently.

Type	Bytes	Range (Signed)	Range (Unsigned)
TINYINT	1	-128 to 127	255
SMALLINT	2	-32768 to 32768	65535
MEDIUMINT	3	-8388608 to 8388607	16777215
INTEGER	4	-2147483648 to 2147483647	4294967295
BIGINT	8	-9223372036854775808 to 9223372036854775807	18446744073709551615

If you're wondering from where those range values come from, here's an exercise.

TINYINT (1 byte/8 bits)

1	1	1	1	1	1	1	1
27	26	25	24	23	22	21	20
128	64	32	16	8	4	2	1
Total							255

When the value is unsigned, the range goes from 0 to 255 (a total of 256 possible values, and 0 is one of them). If the value is signed, the range is divided in half (128), but we need a slot for 0 since the total possible values is still just 256. If we said -128 to 128, counting zero, we would need 257 possible values (because we're not talking about the digit value, but what it needs to be in memory, so 128 negative numbers + 128 positive numbers + 1 for 0), causing an overflow.

So we consider the 0 a non-negative value, making the range from 0 to 127 a total of 128 possible positive values, therefore, the range for negative values goes from -128 to -1 making up another 128 values, using the total 256 values available.

Joins

Joins are used to connect tables based on supplied criteria. This lets you retrieve information from related tables. For the following examples, we're going to use these tables:

The customers table (left):

u_id	u_name	city_id
1	John Doe	1
2	Mary Doe	2
3	Kevin Doe	1
4	Kelly Doe	NULL

The cities table (right):

c_id	c_name
1	San Salvador
2	San Vicente
3	Soyapango
4	Apaneca

There are multiple types of joins.

Inner Join

This is the default type of join in MySQL (hence, the INNER in INNER JOIN is effectively optional). This type will join the records that match in both tables and exclude anything that does not match.

When we say match, we're referring to the indicated column value from the left table (customers) that have the same value on the indicated column value from the right table (cities).

SELECT * FROM customers INNER JOIN cities ON customers.city_id = cities.c_id

Like we said earlier, the INNER keyword is optional since, at least in MySQL, it is default behavior:

SELECT * FROM customers JOIN cities ON customers.city_id = cities.c_id

The result will be:

u_id	u_name	city_id	c_id	c_name
1	John Doe	1	1	San Salvador
2	Mary Doe	2	2	San Vicente
3	Kevin Doe	1	1	San Salvador

We only got records where both tables match the given column value. In other words, where the city ID of a record in customers matches a record in city. Effectively, we didn't get Kelly Doe because there's no city with ID NULL.

Left Outer Join

This type of join retrieves all the records of the left table (customers) even if they don't match any record on the right table (cities). But still, it will retrieve only the matched records of the right table.

The OUTER keyword, just like INNER, is optional when using LEFT JOIN since it's the default behavior.

SELECT * FROM customers LEFT OUTER JOIN cities ON customers.city_id = cities.c_id

Or:

SELECT * FROM customers LEFT JOIN cities ON customers.city_id = cities.c_id

The result will be:

u_id	u_name	city_id	c_id	c_name
1	John Doe	1	1	San Salvador
2	Mary Doe	2	2	San Vicente
3	Kevin Doe	1	1	San Salvador
4	Kelly Doe	NULL	NULL	NULL

We got Kelly Doe included in the result set, even though no city matches the query, therefore all the columns related to cities are NULL.

Essentially, we're saying "get the records that match in customers and cities, nonetheless, get everything from customers".

Right Outer Join

The opposite to the right join, this one retrieves all the records of the right table (cities) even if they don't match any record on the left table (customers). But still, it will retrieve only the matched records of the left table.

The OUTER keyword, just like INNER, is optional when using RIGHT JOIN since it's the default behavior.

SELECT * FROM customers RIGHT OUTER JOIN cities ON customers.city_id = cities.c_id

Or:

SELECT * FROM customers RIGHT JOIN cities ON customers.city_id = cities.c_id

The result will be:

u_id	u_name	city_id	c_id	c_name
1	John Doe	1	1	San Salvador
3	Kevin Doe	1	1	San Salvador
2	Mary Doe	2	2	San Vicente
NULL	NULL	NULL	3	Soyapango
NULL	NULL	NULL	4	Apaneca

We got Soyapango and Apaneca added to the result set, even though no customer is referencing those cities, therefore, the customer-related fields are NULL. Also, we didn't get Kelly Doe this time, because there's no city where the ID is NULL, so that's not a match.

Essentially, we're saying "get the records that match in customers and cities, nonetheless, get everything from cities".

Full Outer Join

This type of join is the mix of the left and right joins. It will retrieve all the records, either match or not, from the left table and the right table.

SELECT * FROM customers FULL OUTER JOIN cities ON customers.city_id = cities.c_id

Or:

SELECT * FROM customers FULL JOIN cities ON customers.city_id = cities.c_id

The result will be:

u_id	u_name	city_id	c_id	c_name
1	John Doe	1	1	San Salvador
2	Mary Doe	2	2	San Vicente
3	Kevin Doe	1	1	San Salvador
4	Kelly Doe	NULL	NULL	NULL
NULL	NULL	NULL	3	Soyapango
NULL	NULL	NULL	4	Apaneca

It contains all the records from both tables, either they match or not, but those who match will be related in the result set as seen above.

Full Outer Join for MySQL

FULL OUTER JOIN does not exist in MySQL 😨 but they can be emulated 😌. Using the previous example, we can MySQL-it as follows:

(
    SELECT *
    FROM customers
    LEFT JOIN cities ON customers.city_id = cities.c_id
)
UNION ALL
(
    SELECT *
    FROM customers
    RIGHT JOIN cities ON customers.city_id = cities.c_id
)

We apply UNION ALL (so duplicates are not removed, just like FULL JOIN) to the result of the LEFT JOIN and RIGHT JOIN.

Keys

Keys impose constraints on columns. There are multiple types of keys, but to mention a few of the most used:

• UNIQUE: Ensures that all values in the column are different. You can have many UNIQUE constraints in one table.
• PRIMARY: Can be defined on either a single or multiple columns. It guarantees that each row in the database will have a unique value combination for the columns in the key, hence automatically using UNIQUE. However, you can have only one PRIMARY KEY constraint. A row may not have a null value for its primary key.
• FOREIGN: It references a primary key of another table.

Indexes

Indexes are data structures optimized for lookups and help the database find results faster. MySQL requires every key also to be indexed, therefore, at least in MySQL, keys and indexes are synonyms. This requirement is to improve performance.

Note: Although the above implies that you cannot have keys without indexes, it is possible to index columns that are not keyed.

The way indexes work is by creating an "alternative database" (not called like that, but it's easier to visualize it this way) using the B-tree (for PRIMARY KEY, UNIQUE, INDEX, and FULLTEXT) data structure with the indexed columns data.

This way, when you query data, the query plan will determine the best way to query the data. If the query contains any of the indexed columns, the engine will prioritize the index, therefore it does not need to go through all the rows and columns. Of course, the above depends on the query, but in a nutshell, that's how it works.

Think of indexes as a table of contents. You don't go through an entire GitHub README file to read about MySQL indexes; instead, you click the burger and search through the titles of the README 😄

The syntax for creating indexes is:

CREATE INDEX index_name
ON table_name (column_name, ...);

Let's create an index for the c_name column of the customers table we were using earlier:

CREATE INDEX customers_name_idx
ON customers (c_name); -- There can be multiple columns inside the parentheses.

Now if we create a query like this:

SELECT COUNT(*)
FROM customers
WHERE c_name = 'John Doe'

The engine will use the index to find the record.

SELECT COUNT(*)
FROM customers
WHERE c_name = 'John Doe'
    AND city_id = 1

The engine will still use the index because the query plan is very smart 🤓 and will notice that we're querying an indexed column, and the result set will contain only records that match the condition, so, it will reduce the query scope to only those records instead of the whole database. Pretty neat, huh? 😃

Note: Indexes need to be updated when data is changed, this adds overhead to writing. So use indexes wisely 🧐

XML

XML (or eXtensible Markup Language) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. It is a subset of the Standardized General Markup Language (SGML).

PHP has 2 type of XML parsers:

• Tree Parsers: These attempt to parse the entire document at once and transform it into a tree structure.
  • DOMDocument
  • SimpleXML
• Event-based Parsers: These work by reading the XML document node by node and providing you the opportunity to hook into events associated with this reading process.
  • XMLReader
  • XML Expat parser

Building XML

DOMDocument

$domDocument = new \DomDocument();
$tasks = $domDocument->createElement('Tasks');

$task = $domDocument->createElement('Task', 'Add the SimpleXML version of this example.');
$task->setAttribute('required', 'true');

$tasks->appendChild($task);
$domDocument->appendChild($tasks);

echo $domDocument->saveXML();

Full example: src/XML/DOMDocument/Build.php

▶️ Run exercise:

php trainer xml:dom:build

SimpleXML

$tasks = new \SimpleXMLElement('<Tasks></Tasks>');

$task = $tasks->addChild('Task', 'Notify the reader to check the full example');
$task->addAttribute('required', 'true');

Note that SimpleXML is designed after its name, to be simple. It has limitations for which you can rely on the DOMDocument class. Check the full example for more information.

Full example: src/XML/SimpleXML/Build.php

▶️ Run exercise:

php trainer xml:simple:build

XPath

XPath is a major element in the XSLT (eXtensible Stylesheet Language Tranformations) standard. It can be used to navigate through elements and attributes in an XML document.

You can use XPath expressions directly in DOMDocument and SimpleXML.

DOMDocument

$domDocument = new \DomDocument();
$domDocument->load('library.xml');

$xpath = new \DomXpath($domDocument);
$results = $xpath->query("//CD/YEAR");

foreach ($domNodeList as $element) {
    echo '[' . $element->nodeName . '] ';

    foreach ($element->childNodes as $node) {
        echo $node->nodeValue . PHP_EOL;
    }
}

Full example: src/XML/DOMDocument/XPath.php

▶️ Run exercise:

php trainer xml:dom:xpath

SimpleXML

$xml = simplexml_load_file('library.xml');
$results = $xml->xpath("//CD/YEAR");

foreach ($results as $element) {
    echo '[' . $element->getName() . '] ';

    echo $element . PHP_EOL;
}

Full example: src/XML/SimpleXML/XPath.php

▶️ Run exercise:

php trainer xml:simple:xpath

SOAP

PHP ships a SOAP client and a SOAP server with it. You can make calls to web services and map them to object members (properties and methods).

$client = new \SoapClient('https://www.dataaccess.com/webservicesserver/NumberConversion.wso?WSDL');

$request = ['ubiNum' => 15665];
$response = $client->NumberToWords($request);

echo $response->NumberToWordsResult;

Full example: src/SOAP/Client.php

▶️ Run exercise:

php trainer soap:client
php trainer soap:client 42