"today I downloaded Zend Development Environment 5.0 beta to give it a try. I've been using ZDE since version 3.0, and in fact developed all of PEAR 1.4 in ZDE. The server debugger has made it extremely easy to debug complex apps like PEAR_Frontend_Web that have multi-page interactions. It's also been good when trying to develop applications that run on linux and windows, and especially with debugging PEAR's unit tests."
(Via Planet PHP.)
I'd definitely look into this, if it didn't cost an arm and a leg!