Deprecate returning non-string values from a user output handler

[DanielEScherzer]

https://wiki.php.net/rfc/deprecations_php_8_4

[Girgias]

Thank you for tackling this! I added a few comments and I think the reason you don't get the "correct" name for anonymous functions should be fixed with #19011

[DanielEScherzer]

Thank you for tackling this! I added a few comments and I think the reason you don't get the "correct" name for anonymous functions should be fixed with #19011

Rebased after that patch was merged and indeed the anonymous functions no longer just say "Closure::__invoke", 87756d7

[Girgias]

Few nits, but looks mostly good :)

[bukka]

There was reason why we didn't merge this: See https://externals.io/message/124506#124666 . @DanielEScherzer have you considered all those edge cases?

[bukka]

I think this should get another RFC that will in more detail consider output handler behavior.

[bukka]

Or lets leave it as you put already some effort into this and we will see how it goes. It was strange edge case anyway and might be a bit unusual for people to do that.