If you looking for something like a fully fledged IDE, give Eclipse PHP a try. It has a ton of addons/modules available to add to it's already great capabilities. If you're looking for something more simple, perhaps have a look at Komodo Edit, Programmer's Notepad or Notepad++.
I currently use Komodo Edit which has a very simple interface and very basic functionality. It has tabbed editing of multiple documents and syntax syntax highlighting which is all I really need.