On the PHPMagazine.net site there's a new article about PHP user groups and some of the resources that can be used to help find one in your area.
Part of this community a lot of core developers, documentation editors, web application developers, hackers, Quality assurance team, tons of open source projects, PHP UG around the world, and many many other people contributing each one in its category, and even friends and family! Users Group are an active element of the PHP ecosystem, and I believe that everyone should be involved to contribute and keep these UG active around the world.
Some of the efforts that he's pointed out from the past are the phpgroups.org site, phpusergroups.com as well as the sharemymap.com effort to map the groups out. Unfortunately there's no single resource that you can point to that has a full listing.