I just tried Zend's IDE.
It's quite OK, but the editor part of the IDE is quite primitive:
Auto-indentation is not good.
Code re-formatting is very bad.
No way to trim trailing spaces.
PHPDoc support is very limited.
Can any of you recommend something better?
I wish there were something like IntelliJ IDEA for PHP. IDEA is the most amazing IDE I've ever used. It's only for Java though.