Restrictions...

Hello, I am currently making a File Manager for hosting (I have a reseller account with my host) but I have a problem. I am new to PHP and still learning. I am going to create a user system so I don't have to setup the File Manager for each account as the way I am currently doing. What I am doing is uploading the File Manager script that I wrote into their directory/folder, but I need to find a way to restrict the users so that they can't upload files and delete files just by changing a bit of the script to do this. My script is/will be on 2 servers, one is Unix and other is Windows. A response with your help would be greatly appriciated. Thank you, Brandon Crowl