User upgrade PHP

[iGloo']

I'm not sure how to say it, but let's try.
There is a default option wich allow user to use Paypal for upgrade, but I would like to use something else called Starpass.

I know that I will have to create a PHP page when the codes they entered on the Starpass script are good to move my users to a certain "usergroup". Else, there is a default error.php page.

The question is, how could I make this PHP page to add my users who paid to another usergroup? Like, how the code will look?
This is a basic question I think, but i'm totally noob with that.

Thank you in advance.

 

[iGloo']

Nobody can tell me what kind of query it will be?
The fact is that if my users put the correct codes, they will be redirected to a *.php page. I would like to move those users to another usergroup if they put correct codes.
Thank you,