class PhpRequirements

final class PhpRequirements {
    
    /**
     * The minimum PHP version requirement for the installed Drupal version.
     *
     * This property is maintained to make the class testable.
     *
     * @var string
     *
     * @see version_compare()
     */
    private static $drupalMinimumPhp = \Drupal::MINIMUM_PHP;
    
    /**
     * The expected PHP version end-of-life dates, keyed by PHP minor version.
     *
     * The array keys are in 'major.minor' format, and the date values are in ISO
     * 8601 format.
     *
     * @var string[]
     *   An array of end-of-life dates in ISO 8601 format, keyed by the PHP minor
     *   version in 'major.minor' format. The list must be sorted in an ascending
     *   order by the date. Multiple versions EOL on the same day must be sorted
     *   by the PHP version.
     */
    private static $phpEolDates = [
        '7.3' => '2021-12-06',
        '7.4' => '2022-11-28',
        '8.0' => '2023-11-26',
        '8.1' => '2024-11-25',
    ];
    
    /**
     * This class should not be instantiated.
     */
    private function __construct() {
    }
    
    /**
     * Dynamically identifies the minimum supported PHP version based on the date.
     *
     * Drupal automatically increases the minimum supported PHP version from
     * \Drupal::MINIMUM_PHP to a newer version after PHP's documented end-of-life
     * date for the previous version.
     *
     * Below this version:
     * - New sites can be installed (to allow update deployment workflows that
     *   reinstall sites from configuration), but a warning is displayed in the
     *   installer that the PHP version is too old (except within tests).
     * - Updates from previous Drupal versions can be run, but users are warned
     *   that Drupal no longer supports that PHP version.
     * - An error is shown in the status report that the PHP version is too old.
     *
     * @param \DateTime|null $date
     *   The DateTime to check. Defaults to the current datetime (now) if NULL.
     *
     * @return string
     *   The minimum supported PHP version on the date in a PHP-standardized
     *   number format supported by version_compare(). For example, '8.0.2' or
     *   '8.1'. This will be the lowest PHP version above the minimum PHP version
     *   supported by Drupal that is still supported, or the highest known PHP
     *   version if no known versions are still supported.
     *
     * @see version_compare()
     */
    public static function getMinimumSupportedPhp(?\DateTime $date = NULL) : string {
        // By default, use the current date (right now).
        $date = $date ?? new \DateTime('now');
        // In case no data are available or all known PHP versions in this class
        // are already end-of-life, default to the version that had the most recent
        // end-of-life (the key of the last element in the sorted array).
        // The string cast ensures the value is a string, even if the PHP EOL date
        // array is empty. As of PHP 8.1, version_compare() no longer accepts NULL
        // as a parameter; empty string must be used instead.
        $lowest_supported_version = $lowest_supported_version ?? (string) array_key_last(static::$phpEolDates);
        // Next, look at versions that are end-of-life after the current date.
        // Find the lowest PHP version that is still supported.
        foreach (static::$phpEolDates as $version => $eol_date) {
            $eol_datetime = new \DateTime($eol_date);
            if ($eol_datetime > $date) {
                // If $version is less than the previously discovered lowest supported
                // version, use $version as the lowest supported version instead.
                if (version_compare($version, $lowest_supported_version) < 0) {
                    $lowest_supported_version = $version;
                }
            }
        }
        // If PHP versions older than the Drupal minimum PHP version are still
        // supported, return Drupal minimum PHP version instead.
        if (version_compare($lowest_supported_version, static::$drupalMinimumPhp) < 0) {
            return static::$drupalMinimumPhp;
        }
        // Otherwise, return the lowest supported PHP version.
        return $lowest_supported_version;
    }

}