I don't like those bloatware IDEs like Eclipse etc. It's just too oversized for most of my projects. My favorite tool for writing code is vim (Vi iMproved). And it comes with so many neat features, like code completion, for example.
I just stumbled upon some ways to use code completion for writing PHP code:
When in Insert mode, type the beginning of a PHP function, for example
Then hit <Control-X> <Control-O> and vim will show you all possible functions in a drop down menu. Just choose the one you were looking for and hit enter
Find out more about vim secrets: http://www.vim.org/tips/tip.php?tip_id=305