Prices Depending on referrer

[Skizot]

I need to make a shop that stores an id in the session variables and marks up prices based on this.

 

and there will be many different ID's so if there's an admin edit page, or better yet a way for them to log in and edit their mark up would be even awesome.

 

also, it needs to redirect to a "Please enter a store ID" page if they did not come from a referrer.

 

 

if this doesn't exist could someone point me in the right direction as to what i would have to do (kinda new to php, but an ace at asp) and also, a way to watermark ONLY the images that are on sale.