PHP Mac IDE - Grouping open files

Hi,

I currently use Smultron for developing, it's fine, although the one thing that I really do need is a feature where I can group the files I have open into categories, i.e.

categories.php
class.categories.php

Becomes
Folder / Category thing
-> categories.php
-> class.categories.php

I'd need to be able to name the of Folder/Category ideally.

Really causes me a head ache when I'm editing so many files at once, and need to switch between the files that relate to each other.

Anyone know an app that will let me do this?

Ryan