Okay, there’s a patch to ctags that you could try, but in my experience, it’s not very good. There’s also eclim, but who wants to get that up and running? It’s a huge pain in the neck.
Developer tooling for PHP hasn’t really received too much attention, in my opinion. In fact, decent autocompletion for PHP doesn’t exist outside of heavy IDEs and lately, I’ve seen a lot of people switching away from vim or whatever editor they normally use to PhpStorm or similar simply for code completion, myself included. I miss vim so much that I’m working on an autocomplete library for PHP, and I released a preview of it today. I’ve been calling it “The Force” (though I’m open to changing the name).
It doesn’t do much right now other than index your code, but I’ve got a roadmap and a pretty good idea for how to handle most of it. I’d love for some more people to get involved. You can find the library over in the Github repository.