There have been a few threads on this. These are some of the most popular:
1) Textpad
2) UltraEdit
3) EditPlus
Personally, I prefer
PHPEd - it was designed specifically for PHP, it also allows you to install PHP on your machine to be able to preview your scripts. There are a few quirks in it, like it's not as customizable as I'd like, but for me it works better than any of the others.
Hope this helps
-jim