Verify text from array then modify group

Hello Everyone!

So I'm trying to make an "upgrade" feature which checks if the entered code is correct from a list of array(or a text file) and then modify the user group if the entered code is correct.

Basically, you purchase a code from PayIvy for example and enter it on a little dialog box on the forum itself and do stuff. Just like NBB :yus:

The problem is, since we can't use PHP on MyBB templates, I can't find a way to get this done.

Pretty sure my English is messed up here.