How to fix 403 in Mac OS X built-in Apache?
I have an alias specified in OSX server pointing to a user directory.
I spent a long while chmodding and messing with _www user, adding executable permissions recursively, uninstalling macports and all sorts of stuff trying to get this to work. No idea why it wasn't working.

Eventually, I just checked the "shared folder" checkbox in the Finder for that folder, and it worked, on the specified domain, with php active, the way I wanted it to. :/
...so that was easy.