Nobody mentioned Kate. It's easier than vi for start (and has nice vi-mode for those, who want to migrate to vi), has more options than gedit (And better syntax highlighting). It also has kioslaves support (nice for remote server PHP development) and it's only a little bit more CPU-demanding than gedit. It can also have built-in console (extremely helpful if you want to quick grep through files or compile the project).
There are also features like:
- basic code completion
- advanced indentation and block selection operations
- good and very clean (to read) find/replace with regexp
- comment-out on ctrl+d (it comments out one line or one function if used on function header)
and a lot more...
