Description

While researching within the PHP source codebase on Enums, I encountered several inconsistencies for which I've been unable to discover the reason.

Magic Method Errors

Enums cannot implement any magic methods, besides __call, __callStatic and __invoke. Implementing others will give you an E_COMPILE_ERROR of Enum may not include %s. But, because of how __sleep, __wakeup and __set_state are handled, implementing them gives you an E_COMPILE_ERROR of Enum may not include magic method %s. While I don't entirely understand why there's a different code path for those magic methods, I believe the difference is an internal implementation issue; therefore, the error messages for the end user should be consistent.

I'm pretty confident the solution for this would be to update the error message thrown by ZEND_ENUM_DISALLOW_MAGIC_METHOD, as the second error message that mentions magic methods is nicer and more descriptive. I do not know the impact of this, as I'm unsure where error messages fall when it comes to backwards compatibility.

I am happy to test and PR myself.

Banned Interfaces

Enums can implement any interface, except Serializable, which I suspect is down to the fact that this interface cannot be implemented without also implementing some of the magic methods that enum cannot have. There is a specific code path for this that throws an error.

I believe that the Stringable interface falls into this same category. Since this interface has a single method, __toString, it cannot be implemented without using one of the banned magic methods.

I think the best solution would be to add a specific check for Stringable within zend_verify_enum_interfaces. However, I'm unsure of the side effects as Stringable is both an implementable and an implicit interface.

I am happy to test and PR myself.