[php-langspec][Issue #251] - The optional method and properties overriding rules are required.

You can view the Issue on github:
https://github.com/php/php-langspec/issues/251

Opened By: ShenoudaGergis
Issue Description:
The specification mentioned that if the *overriding* method is not compatible with the overridden method, a **non-fatal** error will be issued. But for `PHP 8 >` the error is **fatal**.

Also the rules for properties overriding are not mentioned.

Thanks for the great work.