[RFC][Discussion] Make compact function reports undefined passed variables

Hello dear internals, how are you? I'd like to propose a new RFC to PHP's core, but as this one contains a BC Break, let's discussed it before making anything official. A couple of days ago, while discussing some [Coding Standards rules for Doctrine, forbidden the compact function]( https://github.com/doctrine/coding-standard/pull/49), an argument caught my attention: > The compact function var does not report undefined variables. Looking in [the compact documentation](https://secure.php.net/compact), this is even emphasizes: > Any strings that are not set will simply be skipped. I couldn't figure out why this is done this way, but, here's what I'd like to propose: make the compact function starts to report undefined passed variables for it. With [only 2 lines of code]( https://github.com/php/php-src/compare/master...carusogabriel:warning-unknow-compact-variable), this is possible, but, of course, this is a BC Break. Let me know your opinion on that, and perhaps, make it happen! Regards, -- Gabriel Caruso