PHP Code:
<a href="http://www.sponsor.com/LongAssReferringURLWithSponsorCode"onMouseOver="window.status='http://www.sponsordomain.com/'; return true;" onMouseOut="window.status=''; return true;">
Your opening link tag should look something like that, and just close the link tag normally (obviously replace both URLs in that tag with your referring URL and the actual domain of the paysite)