Using Subids With Cloaked Affiliate Links

I am sure that many of you have seen the following cloaking code for affiliate links:

<?
} else {
  $a = $_REQUEST['a'];
  if ($a == 1) {
    header("Location: http://www.azoogle1.com?sub=");
    exit;
  }
  if ($a == 2) {
    header("Location: http://www.azoogle2.com?sub=");
    exit;
  }
  if ($a == 3) {
    header("Location: http://www.azoogle2.com?sub=");
    exit;
  }
}
?>

PHP:

You then call each link you want by http://www.pathto.com/script.php?a=1 etc.

My problem is that I want to be able to alter the subid of each URL from the calling URL. So that calling something like this http://www.pathto.com/script.php?a=1?sub=XXXXXX would automatically go to http://www.azoogle1.com?sub=XXXXXX and http://www.pathto.com/script.php?a=2?sub=YYYY would automatically go to http://www.azoogle2.com?sub=YYYY etc.

Willing to pay for anyone who can help me do this, I am sure it is quite simple.

Cheers, Matt.