Heya, Dan.
You're right; PHP-based bug-tracking software is used by all different developers. However, PHP developers are likely to be more familiar with PHP-specific software.
If we don't get enough PHP guys to suggest their favorites, I can always move it to a broader forum.
Any rate, the only other really big PHP-based bug tracker that I'm aware of is phpBugTracker (
http://phpbt.sourceforge.net/).
I don't have much experience with any of the major systems, as my day job is corporate and I generally roll my own apps for contract work.
Good luck with your search!