Havard Eide shares a tool he originally created to do function lookups when in vim - phpm:

4 years ago I wrote a small program called phpm that I used for php function lookup when coding in vim. since then I've crossed over to ZendStudio and I'm now using that for my everyday work. But: every now and then I've had requests regarding phpm and if I can't digg up the sources, and I've said: "sorry, but those are lost". Until today: I went through a old HD, digging through backups and files: and suddenly phpm resurfaced!

He's included the links to the three things you'll need to get it working - the phpm file itself, XML required by it and an optional phpdocumentor template to create the output file.