A little help with form verification

I have a little email signup at the bottom of apage where I want users to put in a confirmation url as proof they went to an affiliate site. How do I check the field where they put in the url that it contains https://register.i-say.com?

The fields on my form are strange cuz they are div's instead of regular old form fields and I don't really understand how to error check them... Please see my page at http://section8facts.com/2009/01/19/the-explosive-truth-about-the-section-8-application-process/

Please put specific code as possible as i'm a newbie. Thx.

 

It does use a form, that field's name is AffiliateCode according to the source.

Something like this:

<?php if ($_POST['AffiliateCode'] == "https://register.i-say.com") {
    // Do something here
} else {
    // Do not do anything here. Give an error
}

Code (markup):