I see that there are plans to add PHP support in IntelliJ 9. Two words: THANK YOU!
I would suggest integration of a debugger and unit testing right away for PHP. In other PHP IDE's, you have to spend a whole day just trying to get the debugger working....and it's usually flaky. Also, for some reason unit testing support is difficult to find in PHP IDE's. It seems like unit testing should be an essential tool for ANY real software developer/engineer.
I'm often stuck working with PHP instead of Java on the server side. It's a huge pain to switch IDE's all the time. Full PHP support (with debugger and unit testing) would be fantastic! Who's with me on this?