Aptana (yes, this is a shameless plug) released version 1.0 of its PHP support yesterday, and it is something that you should definitely check out. Prior to working at Aptana, I was an avid user of Zend Studio, and Coda, simply because those were the best PHP IDEs as far as I was concerned. Over the past few months, however, we've been hard at work on the PHP support for Aptana Studio, and I can say with much confidence that it is in every way as good as every other PHP IDE out there, and better than some in other respects.

He talks about new features like content assist (adding userland code to the automatic completion), completion of functions with parameters already filled in and a PHP preview server and debugger built right in. You can get more information on the Aptana website.