[php-src] Issue #12639: `__toString` LSP is not checked since PHP 8.0

Issue: https://github.com/php/php-src/issues/12639 Author: mvorisek ### Description The following code: ```php <?php interface StreamInterface { public function __toString(): string; } class X implements StreamInterface { public function __toString() { return 'x'; } } ``` Resulted in this output: ``` ``` But I expected this output instead: ``` Fatal error: Declaration of X::__toString() must be compatible with StreamInterface::__toString(): string ``` repro: https://3v4l.org/XfgXZ with any other method the LSP is checked/required ### PHP Version 8.0+ ### Operating System any