PR 287 - added use_keys argument to array_filter() [Discussion]

I'm starting up a thread for discussion on Pull Request 287 https://github.com/php/php-src/pull/287 (allowing array keys to be passed to the callback function of array_filter through a third optional boolean argument). I would like to merge this into master and as discussed on IRC it would probably be a good idea to startup a discussion and make sure there aren't any objections or clarifications not yet voiced. The patch has no BC because the third argument is optional and defaults to false. Personally, I have always thought it would be a good idea to be able to get the keys into the array_filter callback since I've stumbled across a few scenarios where that would have made things easier. I'm not sure if there are any particular down sides to this option being added, but none that I can find. It currently passes all tests in master and works as expected. Thoughts, opinions, objections, concerns? - Sherif