I'm partial to ConTEXT (http://context.cx/) myself. You can download and update different highlighters, so both your HTML and PHP can be syntax highlighted (If the HTML is not in quotes).
It also can save a backup file upon saving, so if you make changes, and say "OH SHOOT!" you've got a chance of retrieving some code you removed.
ConTEXT is also flexible enough to adapt to different programming environments. When we were using the Pep/7 software machine in school, I set ConTEXT up to not only highlight the Pep/7 assembly language, but to assemble and run the Pep/7 object file.
I've also found the global search and replace option very useful. I can open all the files in a project, and then do a search and replace in all the files, not just the one.
Disclaimer: I'm not affiliated, I'm just really happy with the program.
Aptana is probably the easiest for projects. However I sometimes use Notepad++ for day to day small scale stuff.
I usually use Notepad++, but I've had a few people recommend Aptana to me. It looks pretty good, some nice features to it (like code assist, which is nice for lazy people like me =P), I just haven't had the patience to learn a new IDE.