I am currently using Zend Studio 2.5 for debugging applications. It is a little weak on some editor features but it has some nice ones too, for example it will auto complete your user functions as you type them even if the function is located in a seperate file. It will show a brief description of what the function is in a pop up tool tip provided that you use phpdoc. Keeps track of all classes, fields and methods used in a project. It has syntax highlighting. Some things it lacks are code templates and autocomplete for your own snippets. (NOTE it has autocomplete for functions).
I use the Zend Studio in combination with Homesite 5. I think they work great together. The Zend Studio ships with a debugger and so far together the studio seems to be the most relable and easy to use when it comes to debugging applications.
So far I have used PHPedit (phpedit.com) and Maguma Studio Light (maguma.com) both are integrated with an external debugger but they crash alot and have a lot left to be desired in the way of debugging. Although phpedit is a nice editor. I found myself bouncing back and forth between phpedit, maguma, html-kit and homesite and keep coming back to homesite. Also I use dreamweaver for page layout.
