Eclipse isn't just for Java development. You can also download the...

Eclipse PHP Development Tools (PDT)

...which provides PHP development tools for the Eclipse platform.

I've downloaded it but haven't tried it because, well, because it is so darn gigantic (118MB for a Windows download and 117MB for Mac OS X that includes the complete Eclipse installation) like most Java applications seem to be. And, before you ask, yes, I consider Microsoft Visual Studio to be gigantic too. If someone could point me to a good review where the Eclipse PDT features are described in the context of a typical PHP developer, I'd appreciate it. I generally find vi, npad, or EMACS plus grep in a terminal window sufficient for my needs. That's also how I write Ruby code.