I have two favorites which alternate depending upon my mood, and seriousness about my project. I have both phpDesigner 2007 and ConTEXT (free). I like the first when I want code complete, inline help and tooltips, and realtime/inline debugging. And I like ConTEXT when I've no need or want for that stuff. Both have syntax highlighting. ConTEXT can be configured to debug a PHP script by a "user command" functionality, and show the bugs in a result window below the editor.

Like I said, depends upon my wants.

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Programmers are in a race with the Universe to create bigger and better idiot-proof programs, while the Universe is trying to create bigger and better idiots. So far the Universe is winning. - Rich Cook