Found this nifty utility through Digg.com: phpFunctions.
This is a simple Ajax script which works just like Google Suggest, except it looks up PHP functions. Type the first part of a function, and it will automatically find the functions that match, and when you've found the function you're looking for, it'll automatically display all the details (like arguments, description, etc).
Seems like a pretty cool use of Ajax, but I doubt I'll be using it a lot (or even at all). I find going to php.net/[function name] a lot easier and faster. It'd be pretty neat though if this could be built into some web based PHP IDE.