PHP: use enums available since PHP 8.1
PHP 8.1 introduced enums to the language - see https://www.php.net/releases/8.1/en.php#enumerations. It would be nice to use them in the generated code. For our use case, [backed enumerations](https://www.php.net/manual/en/language.enumerations.backed.php) seem appropriate.

PHP 8.1 is currently the oldest version that is not yet EOL (see https://endoflife.date/php), so we don't need any command-line option to activate them and can generate them unconditionally.