If anyone else is interested, I did this to solve my problems.
Perhaps it'll help you, too.
PHP Code:
function getAffiliate($_NATS) {
list($campaignID) = explode(":", base64_decode($_NATS));
open_db();
$sql = "SELECT `a`.`username` FROM `accounts` as `a`,`campaigns` as `c` WHERE `a`.`loginid` = `c`.`loginid` AND `c`.`campaignid` = '$campaignID'";
$result = mysql_query($sql);
$row = mysql_fetch_assoc($result);
close_db();
return $row['username'];
}