When features get added to PHP there's a lot of discussion first about the new functionality. This is done on the so-called internals mailing list. You can try to follow the discussions via a rather ugly interface at http://news.php.net. The site looks like a very old school web email client where all conversations are just running through each other.
A couple of days ago Matthieu Napoli launched his new site externals.io. This site makes following internals a lot easier. Messages that are part of a conversation are grouped and thus much easier to follow. If you're interested in this you should definitely also check out Made with Love's Why We Can't Have Nice Things project that lists all RFC's and votes.