I've been looking for one, but haven't been able to find anything that's even close.
Edit: I really would prefer something that's completely written in PHP, with a web interface rather than a command line. It doesn't have to have all the bells and whistles.
I don't quite understand the question. Do you mean you need a Source Control written in PHP with a GUI? Or you need to do source control on a project written in PHP?
If the second then you could try git, or subversion. They are both open source, and I'm pretty sure you can get decent GUIs for them as well.
I think what the questioner wants is a web-interface to (say) git or SVN, that lets you make commits. Most of the VCS web-interfaces are to browse the history and view commits, not actually change anything.
Assuming I'm right, I don't think you'll find much that does this. The closest I know is Github - it lets you edit a file, and commit that change on the www.github.com repository view. It's pretty basic (it's intended for quickly editing files, nothing more), but easy to use (click "edit", make the change, click "save" then enter a commit message)
However, there are many GUIs for subversion (I'd recommend TortoiseSVN for Windows, Versions or svnX for OS X, not too sure about Linux, I always just used the svn command), git comes with one (run git gui). All of them allow you to make commits and such, and should be a bit more intuitive than the command-line equivalent.
Try ASVCS at http://asvcs.com/ it's a great version control system written out completely in PHP & mySQL.