Over the years our company has been looking for a standard IDE for PHP development. After using Eclipse (PDT) (slow), Aptana (basically Eclipse with additional add-ons and more user friendly), Zend IDE and NetBeans, we decided to give PhpStorm a try.
We never looked back. PhpStorm is simply the best PHP IDE at the moment. Its feature set is *very* complete. Pricing is good (both 'professional' and 'personal' licenses). And the developer is very responsive via their issue tracker, which allows up-voting feature requests and allows for discussing bugs, features etc.
A shortlist of plusses that come in mind:
* Code inspection; Check/validate your code (single file or whole project) and get early warnings for possible errors in your code.
* Integration with various VCS; SVN (1.6 and 1.7), GIT, CVS
* Integration with issue trackers allows automatically grouping changes to change lists connected to your issues
* Direct access to your database for quickly querying and making changes to database schemas
* Plugin-system to add additional features
* Highly customizable
* Cross Platform (Windows, Mac)
* Quite fast; It's still JAVA-based, but works multi-threaded and a lot faster than Eclipse-based solutions
I would urge any serious PHP/Webdeveloper to give PhpStorm a try. Try to download the trial or, if you need more time to test, download the 'EAP' release (alpha releases, less stable, but they allow for testing longer than 30 days).
If you're looking for a *free* solution, have a look at NetBeans, but for professional use (as mentioned before); Pricing is reasonable and shouldn't be a problem.
Also, if you're looking for the best-looking, simple to use IDE, probably PhpStorm is not for you (CODA might be a better solution). PhpStorm is a professional tool where features and integration are more important than looks and ease-of-use.
To get an idea of features that you'll unlikely find anywhere else, have a look at their blog: http://blog.jetbrains.com/webide/