As of PHP 8.1, while `UnitEnum` & `BackedEnum` cannot be implemented by user-defined classes, it seems they can be extended by user-defined interfaces and eventually be implemented by enums.<?phpinterface TestEnumInterface extends \BackedEnum{ public function foo(): string;}enum TestEnum: string implements TestEnumInterface{ case CASE_1 = 'case 1'; case CASE_2 = 'case 2'; public function foo(): string { return 'bar'; }}?>