Editing Files with PHP

I've looked around at tutorial sites and the like and still cannot find a means of doing this :

I would like an interface for the average joe to log in (only need one account - admin) and then select a file, say...main.php and then edit the content of that without any HTML etc. It's just raw text. He then clicks a button and that new file is overwritten and saved.

The main website is set up as a big template, and then on each page, all that changes is that there is a <? include('main.php'); ?> or something.

This probably isn't very clear, i'll paraphrase:

I want an external interface (in a directory root/admin for example) whereby the "admin" can access something says :

quote

Which page would you like to change?

Index
Photos
Menu
Location etc.

Once he clicks "Index" a page opens up with two text areas, one with the content already there, and one that is empty, the content in the top box is just raw text in a file : main.php for example. The content entered in the box underneath will be the new content that I want to be written to the file main.php and it should overwrite it when clicking a button like - submit.

Ideally, this is passworded and safe because we dont want 1337 h4x0rs hacking a restaurant website do we?

quote RikkusRukkus

If I where you, I'd use a database (mySQL), it's more secure than leaving files on 777 (CHMOD - Able to be written, read and executed by all users) and allow people to put up harmful scripts on the pages.

Your PHP server will be running as the root user, and as such can edit files which don't have their permissions set to 777. You can also use PHP to change the permission settings before and after update if you really need to.