NetBeans Free! Plus, the best functionality of all offerings. Includes inline database connections, code completion, syntax checking, color coding, split views etc. Downside: It's a memory hog on the Mac. Be prepared to allow half a gig of memory then you'll need to shut down and restart.
Komodo A step above a Text Editor. Does not support database connections or split views. Color coding and syntax checking are there to an extent. The project control on Komodo is very unwieldy and strange compared to the other IDEs.
Aptana The perfect solution. Eclipsed based and uses the Aptana PHP plug in. Real time syntax checking, word wrap, drag and drop split views, database connections and a slew of other excellent features. Downside: Not a supported product any more. Aptana Studio 2.0+ uses PDT which is a watered down, under-developed (at present) php plug in.
Zend Studio - Almost identical to Aptana, except no word wrap and you can't change alot of the php configuration on the MAC apparently due to bugs.
Coda Created by Panic, Coda has nice integration with source control and their popular FTP client, transmit. They also have a collaboration feature which is cool for pair-programming.
PhpEd with Parallels or Wine. The best IDE for Windows has all the feature you could need and is worth the effort to pass it through either Parallels or Wine.
Dreamweaver Good for Javascript/HTML/CSS, but only marginal for PHP. There is some color coding, but no syntax checking or code completion native to the package. Database connections are supported, and so are split views.
I'm using NetBeans, which is free, and feature rich. I can deal with the memory issues for a while, but it could be slow coming to the MAC.
Cheers! Korky Kathman Senior Partner Entropy Dynamics, LLC
PDT eclipse from ZEND has a mac version (PDT all-in-one).
I've been using it for about 3 months and it's pretty solid and has debugging capabilities with xdebug (debug howto) and zend debugger.
Komodo is wonderful, and it runs on OS X; they have a free version, Komodo Edit.
UPDATE from 2015: I've switched to PHPStorm from Jetbrains, the same folks that built IntelliJ IDEA and Resharper. It's better. Not just better. It's well worth the money.
Source: Stackoverflow.com