I actually use a script like that on my own admin page. Basically all you do is use php to open any file you want to on your site, display it all in a text area field, then after you make changes, save the new content back to the file. I warn you though, never try to edit the page you are currently on.. 🙂 Its doesnt turn out pretty.
Jason Rottman
Webmaster
Project Manager
www.stardefenders.com