Introduce new ExitCode class

This class contains class constants for the new exit codes, so code referencing the constants becomes more self-documenting.

Closes squizlabs/PHP_CodeSniffer 3696

Author: jrfnl

src/Util/ExitCode.php

@@ -0,0 +1,79 @@
+<?php
+/**
+ * Exit codes.
+ *
+ * Note: The "missing" exit codes 8 and 32 are reserved for future use.
+ *
+ * ---------------------------------------------------------------------------------------------
+ * This class is intended for internal use only and is not part of the public API.
+ * This also means that it has no promise of backward compatibility. Use at your own risk.
+ * ---------------------------------------------------------------------------------------------
+ *
+ * @internal
+ *
+ * @copyright 2025 PHPCSStandards and contributors
+ * @license   https://github.com/PHPCSStandards/PHP_CodeSniffer/blob/master/licence.txt BSD Licence
+ */
+
+namespace PHP_CodeSniffer\Util;
+
+final class ExitCode
+{
+
+    /**
+     * Exit code to indicate no issues were found in the code under scan; or a non-scan request succeeded.
+     *
+     * @var int
+     */
+    public const OKAY = 0;
+
+    /**
+     * Exit code to indicate auto-fixable issues were found.
+     *
+     * @var int
+     */
+    public const FIXABLE = 1;
+
+    /**
+     * Exit code to indicate issues were found, which are not auto-fixable.
+     *
+     * @var int
+     */
+    public const NON_FIXABLE = 2;
+
+    /**
+     * [CBF only] Exit code to indicate a file failed to fix.
+     *
+     * Typically, this is caused by a fixer conflict between sniffs.
+     *
+     * @var int
+     */
+    public const FAILED_TO_FIX = 4;
+
+    /**
+     * Exit code to indicate PHP_CodeSniffer ran into a problem while processing the request.
+     *
+     * Examples of when this code should be used:
+     * - Invalid CLI flag used.
+     * - Blocking errors in the ruleset file(s).
+     * - A dependency required to generate a report not being available, like git for the gitblame report.
+     *
+     * @var int
+     */
+    public const PROCESS_ERROR = 16;
+
+    /**
+     * Exit code to indicate the requirements to run PHP_CodeSniffer are not met.
+     *
+     * This exit code is here purely for documentation purposes.
+     * This exit code should only be used in the requirements check (`requirements.php` file), but that
+     * file can't use the constant as it would block _this_ file from using modern PHP.
+     *
+     * {@internal The code in the requirements.php file and below should always stay in sync!}
+     *
+     * @var int
+     */
+    public const REQUIREMENTS_NOT_MET = 64;
+
+
+}//end class