PhpStorm also provides visual debugging. This allows easy inspection of local variables and user-defined arrays or other objects. You can also use the IDE to perform remote debugging for your server and evaluate expressions in runtime. The debugging works across multiple session and can maintain the session while moving between pages. It works with Zend and XDebug toolbars to debug directly from any modern browser. Duplicate code can be detected and PHPDoc is supported to encourage best-practice documentation of variables and annotating other objects.
PhpStorm is a relatively inexpensive commercial PHP IDE and provides a supporting environment for faster, more accurate development of PHP-based web applications. I have used it for a year now for various projects and it is a definite improvement over using just a text editor and even a smart text editor like Notepad++. The ability to synchronize with a web server over FTP, while maintaining project files locally, is certainly a great feature for rapidly testing different looks and approaches to a page.
You can download a trial, view demo videos, or purchase the software from http://www.jetbrains.com/phpstorm/