Note, to those that wonder: It moved to git.php.net as github only contains a mirror. And other parts of the PHP project are still in transition. There's also a mercurial mirror. The PHP source module is named php-src.
Wow. I think one of the coolest things about this, is seeing a GitHub repo with some of the commits being from 12 years ago. Gives you an idea of how long PHP's been around as compared to a lot of the projects floating around GH.
I think it's very dependent on the project. There really are times in php-src when you want to be able to go back a fair way, since the last significant change to function's implementation may well have been 10 years ago in the early 4.x days (and Git is going to be a huge win for us there over Subversion).
I wouldn't be surprised if it makes much less sense for the kernel, though.
Also, github is just a mirror. The main repo is on git.php.net where they are continuing to use their karma based everyone-commits-to-a-central-repo system (see http://git.php.net/?p=karma.git;a=summary). This goes together with the rest of the infrastructure that they continue to self-host