I found exuberant ctags, a library which can be used to generate an index file mapping language objects to source files and the line in the source file where they are declared. Contrary to its name, it's not just for the C language; it currently supports 33 different programming languages, including PHP.

He also includes a usage example in the post, specifically for getting the tool up and working in your Vim installation (via a bash script). Then it's just a matter of issuing a :set command to point at the correct file and you're one keystroke away from the function/class declaration of your choice.