I'm always amazed that more people don't use ActiveState Komodo.
It has the best debugging facilities of any PHP IDE I have tried, is a very mature product and has more useful features than you can shake a stick at. Of note, it has a fantastic HTTP inspector, Javascript debugger and Regular Expression Toolkit. You can get it so that it steps through your PHP, then you see your Javascript running, and then see your HTTP traffic going out over the wire!
It also comes in free (Komodo Edit) and open (OpenKomodo versions).
Oh, and if you don't always hack just on PHP, it's designed as a multi-language editor and rocks for Ruby and Python too.
I've been a happy customer for around 5 years.
