Changing product price

[zhumar]

We have a client that wants to offer discounts to users that visit his site if they navigate to a certain URL and get redirected. For example, let's say his site is at mystore.com. We set up a redirect for test.mystore.com. If a user goes to test.mystore.com, the user should get redirected to mystore.com and get a 15% discount on all items. I know I should be able to use the referrer URL in PHP to work out some discount calculations. What I don't know is how to update the product price to apply the calculated discount. Is there something I can do to get discounts to work in this way?