If you're running Linux or any other Un*x, nothing can beat Emacs. The latest version has a sexy PHP mode that makes editing a breeze.
I've never tried anything else, really, but I know there are a few graphical PHP editors out there -- janeBuilder is one of the better known ones.
If you're running Windows, I'd try Macromedia's Homesite. It's not free, but it was a really great piece of software back when Allaire owned it.