There's been a fair bit of talk about PHP IDE's of late. That's not surprising given how useful they can be. (Really, folks, vi can only take you so far.) Most of the attention has been focused on the big boys: Eclipse and its derivatives (both free and commercial), Komodo, and NetBeans. Eclipse and NetBeans are both Java based, and Komodo is based on Mozilla's XUL platform (which also runs Firefox and company). I've been bouncing between them for a while, and haven't really been satisfied with any of them. I usually refer to Eclipse as the one I hate least. :-)
There's a new contender to keep an eye on, though, that is worthy of notice: KDevelop.