You could take a look at threads such as
http://ubuntuforums.org/showthread.php?t=6762.
This question has been asked in numerous forms over time, and the answers are all out there.
That said, since I am posting anyway, here are my recommendations:
CakePHP is a framework
hidden cameras based on the MVC (Model View Controller) ideal - recommended, but nothing to do with your question .
Eclipse in general will give you a very fully-featured IDE (I haven't tried the php plugin), as will Quanta +. Bluefish too has very many features.
However, I prefer Geany - also in the repos.
Turkish PropertyIt is much lighter than any of the above IDEs but still supports all the features I have wanted. YMMV.