I'm nitpicking, but they were never using bugzilla but their own system (which now lives here: http://git.php.net/?p=web/bugs.git;a=summary)

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