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

it is a DVCS. it doesnt matter where it's hosted. external contributors should use github, core developers will go through git.php.net. it's all the same.

PHP's always used it's own tracker (http://bugs.php.net).