In another post ( IBM is taking PHP (and Drupal) very seriously ), I mention a series of articles IBM is writing in the context of using Drupal as a web application framework.
In Part 6: Building a custom module in Drupal it is mentioned (and you can see from the graphics) that they are using PHPeclipse in their IDE.
IBM is taking PHP and PHP-powered web applications like the Drupal CMS very seriously.
See for example, the important series of articles Using open source software to design, develop, and deploy a collaborative Web site
This concentrates on a fascinating analysis of Drupal (better in-depth documentation than you can find even on the Drupal website, which naturally celebrates the series of articles), how it works, how to theme it, how to extend its functionality with modules, most of all, a practical example of how to use it!