ReflectionAttribute_getCurrent.php

<?php

#[Attribute(Attribute::TARGET_FUNCTION | Attribute::TARGET_METHOD)]
final readonly class Handler
{
    /**
     * @param ?class-string $messageClass
     */
    public function __construct(
        public ?string $messageClass = null
    ) {}
}

final readonly class HandlerMetadata
{
    public static function fromAttribute(ReflectionFunctionAbstract $function): ?self
    {
        $attribute = $function->getAttributes(Handler::class)[0] ?? null;

        if ($attribute === null) {
            return null;
        }

        $messageClass = $attribute->newInstance()->messageClass;

        if ($messageClass === null) {
            return null;
        }

        return new self($messageClass);
    }

    public static function fromSignature(ReflectionFunctionAbstract $function): ?self
    {
        $messageParameter = $function->getParameters()[0] ?? null;

        if ($messageParameter === null) {
            return null;
        }

        $messageClass = (string) $messageParameter->getType();

        if (!class_exists($messageClass)) {
            return null;
        }

        return new self($messageClass);
    }

    /**
     * @param class-string $messageClass
     */
    public function __construct(
        public string $messageClass,
    ) {}
}

$function = new ReflectionFunction(myHandler(...));

$metadata = HandlerMetadata::fromAttribute($function)
    ?? HandlerMetadata::fromSignature($function)
    ?? throw new \LogicException('Failed to infer metadata for myHandler()');