<?php if($whichsite == "www.domain1.com" || $whichsite == "domain1.com") { ?>
//Replace this with banners for this site
<?php } elseif($whichsite == "sub.domain1.com") { ?>
//Banners code for sub.domain1.com
<?php } else { ?>
//Catch all banner (if you use this add in a non-applicable spot) it will display this code) You could use this alone as a substitue for sub.domain1.com if you are only doing this on 2 sites.
<?php } ?>