Add a simple linter and a pre-commit hook for it

.husky/pre-commit

@@ -1,3 +1,4 @@
+php lint.php
 npm run lint
 composer cs
 vendor/bin/phpstan analyse --memory-limit=1G

lint.php

@@ -0,0 +1,186 @@
+<?php
+/**
+ * Small linter that can detect and fix some code style violations
+ *
+ * Usage:
+ *  php lint.php // Show errors.
+ *  php lint.php --fix // Auto-fix errors where supported.
+ *
+ * @since 3.18.0
+ */
+
+// Configuration.
+$config = array(
+	new Lint_Rule(
+		'Two or more consecutive empty lines found',
+		'/(\r?\n){3,}/',
+		'/\.(php|js|css|html|scss|yml|md|ts|tsx|sh|json)$/',
+		"\n\n"
+	),
+	new Lint_Rule(
+		'One or more empty lines between closing brackets found',
+		'/(\})\n{2,}(\})/',
+		'/\.(php|js|css|html|scss|yml|md|ts|tsx|sh|json)$/',
+		"$1\n$2"
+	),
+);
+
+// Run the rules and display any errors.
+foreach ( $config as $rule ) {
+	$errors = $rule->run( in_array( '--fix', $argv ) );
+
+	if ( '' !== $errors ) {
+		$exit_code = 1;
+		echo $errors;
+	}
+}
+
+exit( $exit_code ?? 0 );
+
+/**
+ * A class representing a linting rule.
+ *
+ * @since 3.18.0
+ */
+class Lint_Rule {
+	/**
+	 * @var string Error message to display when this linting rule is violated.
+	 */
+	private $error_message;
+
+	/**
+	 * @var string Regex pattern used for rule violation detection.
+	 */
+	private $error_pattern;
+
+	/**
+	 * @var string Regex pattern used to specify the files to search in.
+	 */
+	private $include_files;
+
+	/**
+	 * @var string|null Optional regex pattern used to auto-fix the violation.
+	 */
+	private $fix_pattern;
+
+	/**
+	 * @var array<string> Directories to exclude from search.
+	 */
+	private $exclude_dirs;
+
+	/**
+	 * @var array<string> Array of errors found when running the rule.
+	 */
+	private $errors = array();
+
+	/**
+	 * Constructor.
+	 *
+	 * @since 3.18.0
+	 *
+	 * @param string        $error_message The error message to display.
+	 * @param string        $error_pattern The regex matching the rule violation.
+	 * @param string        $include_files The regex matching the files to search in.
+	 * @param string|null   $fix_pattern   The regex to fix any violations (optional).
+	 * @param array<string> $exclude_dirs  The directories to exclude from search.
+	 */
+	public function __construct(
+		string $error_message,
+		string $error_pattern,
+		string $include_files,
+		string $fix_pattern = null,
+		array $exclude_dirs = array( 'artifacts', 'build', 'vendor', 'node_modules' )
+	) {
+		$this->error_message = $error_message;
+		$this->error_pattern = $error_pattern;
+		$this->include_files = $include_files;
+		$this->fix_pattern   = $fix_pattern;
+		$this->exclude_dirs  = $exclude_dirs;
+	}
+
+	/**
+	 * Runs the rule to detect or auto-fix any violations.
+	 *
+	 * @since 3.18.0
+	 *
+	 * @param bool $auto_fix_mode Whether violations should be auto-fixed.
+	 * @return string The violations found, or an empty string.
+	 */
+	public function run( $auto_fix_mode ): string {
+		$base_dir = __DIR__ . '/';
+		$iterator = $this->create_iterator( $this->exclude_dirs );
+
+		// Iterate over the directories and files.
+		foreach ( $iterator as $file ) {
+			// Check if the item is a file that should be checked.
+			if ( $file->isFile() && preg_match( $this->include_files, $file->getFilename() ) ) {
+				$file_content = file_get_contents( $file->getPathname() );
+
+				// Check for rule violations.
+				if ( preg_match_all( $this->error_pattern, $file_content, $matches, PREG_OFFSET_CAPTURE ) ) {
+					$relative_path = str_replace( $base_dir, '', $file->getPathname() );
+
+					// Auto-fix or log the issue.
+					if ( $auto_fix_mode && null !== $this->fix_pattern ) {
+						$fixed_content = preg_replace( $this->error_pattern, $this->fix_pattern, $file_content );
+
+						if ( null === $fixed_content ) {
+							$this->errors[] = "Error while trying to lint {$relative_path}.";
+						}
+
+						if ( false === file_put_contents( $file->getPathname(), $fixed_content ) ) {
+							$this->errors[] = "Failed to save fix for {$relative_path}.";
+						}
+					} else {
+						foreach ( $matches[0] as $match ) {
+							$line_number    = substr_count( substr( $file_content, 0, $match[1] ), "\n" ) + 1;
+							$this->errors[] = "{$relative_path} \033[90m(line {$line_number})\033[0m";
+						}
+					}
+				}
+			}
+		}
+
+		if ( empty( $this->errors ) ) {
+			return '';
+		}
+
+		$output = "\033[31mERROR: {$this->error_message}.\033[0m\n";
+		if ( null !== $this->fix_pattern ) {
+			$output .= "\033[33mAuto-fixable with the --fix parameter.\033[0m\n";
+		}
+		$output .= "\033[36mYou can use the {$this->error_pattern} regex to find the offending code.\033[0m\n\n";
+
+		foreach ( $this->errors as $error ) {
+			$output .= " - {$error}\n";
+		}
+
+		return $output . "\n\n";
+	}
+
+	/**
+	 * Creates an iterator that iterates across the project's folder structure,
+	 * skipping excluded directories.
+	 *
+	 * @since 3.18.0
+	 *
+	 * @return RecursiveIteratorIterator
+	 */
+	private function create_iterator(): RecursiveIteratorIterator {
+		return new RecursiveIteratorIterator(
+			new RecursiveCallbackFilterIterator(
+				new RecursiveDirectoryIterator( __DIR__, RecursiveDirectoryIterator::SKIP_DOTS ),
+				function ( $current, $key, $iterator ) {
+					if ( $iterator->hasChildren() && in_array( $current->getFilename(), $this->exclude_dirs ) ) {
+						return false; // Skip excluded directories.
+					}
+
+					return true;
+				}
+			),
+			RecursiveIteratorIterator::SELF_FIRST
+		);
+	}
+}
+
+// phpcs:ignoreFile