A decent IDE helps a lot when you are working on large object oriented projects or with frameworks like Zend Framework etc, with lots of classes and libraries. PDT has pretty good code completion, has class browsers etc, ctrl+click on a class/method in your code takes you to the file. No need to look things up in the php manual if you forget the order of parameters etc. I use subclipse as well which is useful for version control, gives you a quick display of what files have been modified, need to be checked in etc.