PHP: use enums available since PHP 8.1

[generalmimon]

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 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.