[php-src] Issue #9236: Partially support first-class callable syntax in constant expressions

Issue: https://github.com/php/php-src/issues/9236 Author: pevdh ### Description The first-class callable syntax is currently not allowed in constant expressions: ```php class X { const FN = strlen(...); } var_dump((X::FN)('test')); // PHP Fatal error: Constant expression contains invalid operations ``` I propose partially supporting the first-class callable syntax in a constant expression. Specifically, I propose that PHP allows these two cases in a constant context: ```php class X { const A = strlen(...); const B = SomeClass::someStaticMethod(...); } ``` While (still) disallowing expressions that cannot be known at compile time: ```php class X { const C = $closure(...); const D = $obj->method(...); const E = ($obj->property)(...); // etc. See the first-class callables RFC for a complete list } ``` ## Example Symfony's Choice assertion allows the user to specify a callback. It would be nice if we could use the first-class callable syntax in attributes, but this is currently not allowed: ```php use Symfony\Component\Validator\Constraints as Assert; class User { #[Assert\Choice(callback: Role::values(...))] // Using this syntax here causes an error public string $role; } enum Role: string { case Customer = 'customer'; case Admin = 'admin'; public static function values() { return array_map(fn (Role $a) => $a->name, self::cases()); } } ``` ## Implementation I hacked together [a working patch](https://github.com/pevdh/php-src/commit/343ba93d66b720aa14f058773c023f2ca3326d85) that supports the strlen(...) syntax in constant expressions. I haven't gotten around to implementing SomeClass::someStaticFunction(...) yet. There are probably some edge cases that I'm missing - I'm not too familiar with the PHP code base. I'm also not sure if this feature would require an RFC. I would imagine it would, though. Please let me know what you think.