[RFC] Add get_error_handler(), get_exception_handler() functions

[arnaud-lb]

Currently, the only way to fetch the current error and exception handlers is to push a new one before restoring it:

$current_error_handler = set_error_handler('valid_callback');
restore_error_handler();

This PR adds get_error_handler(), get_exception_handler() functions to address this use-case. They return the last value passed to set_error_handler() or set_exception_handler(), respectively, or NULL if no handler was set.

Related: symfony/symfony#58372 (comment).

cc @lyrixx @nicolas-grekas

[Girgias]

Can't you have a return type of ?callable ?

[arnaud-lb]

Indeed, that's better

[nielsdos]

I believe you can use RETURN_COPY instead of RETURN_ZVAL because those handlers can't be references, so the generated code will be a bit shorter by using RETURN_COPY then.