I edit PHP (among other stuff) with gvim when I've got a GUI, vim (same for console) when I don't. Free, runs on any Unix, Windows, DOS, Mac, Amiga and OSes I have never seen.

It's based on Unix's vi editor, which a lot of people say stands for "Virtually Impossible". It takes a while to get the hang of modal editing, but once you do, you'll feel the powa. There's an 'about vi' page at:

about Vim : vim online

If you're not sure you're ready to move on from notepad-like editors, gvim can do that too:

for non-Vim users : vim online

I hope that helps.