I've been through more IDE's than I can remember, but I may have stumbled onto a winner. When I was doing my development on Windows, I had finally settled on phpEd by Nusphere. Since moving to an Ubuntu development platform, I've been trying out various solutions, such as Jedit and Aptana.
Jedit was just a little too nebulous for me. Aptana was good, but I found the power of its perspective functionality to be a bit too complex, its PHP support to be a little underpowered, its documentation to be lacking, and its future, now that it has been bought out, to be too much in question for me to invest my time in it. Once version 3 is solid and its direction determined, I might look again. I even tried Eclipse, but I make it a point not to use software that makes me feel like an idiot, other than Linux.
The other day I stumbled across a forum entry referring to PhpStorm by JetBrains. It's not Open Source, and neither was phpEd, but with this being the primary tool for my craft, I need to use what works best for me. So far, I find it to be a powerful tool, with all the functionality I require, such as hinting, an through help system, and intuitive support for source control, such as CVS, SVN and GIT. The one hurdle I had to clamber over was that it requires Sun JDK to run, but that's another post.