Adwords - Simple split testing - redirect keeping auto-tagging in place for Analytics

gistfile1.php

<?php

/* bypass special offers for past visitors
must ad ?remark=x to ads' destination url */

if($_GET["remark"]) { 
}else{

/* check to see if adwords sent visit */
if($_GET["gclid"]) { 
      $source= $_GET["utm_source"];
      $term= $_GET["utm_term"];
      $medium= $_GET["utm_medium"];
      $campaign=$_GET["utm_campaign"];
      /* forward visitors to one of three variants while preserving adwords auto-tagging tags */
      switch(rand(1,3)) {
          case 1: header('location: /index.php?utm_source='.$source.'&utm_medium='.$medium.'&utm_term='.$term.'&utm_campaign='.$campaign.'');break;
          case 2: header('location: /a/index.php?utm_source='.$source.'&utm_medium='.$medium.'&utm_term='.$term.'&utm_campaign='.$campaign.'');break;
          case 3: header('location: /b/index.php?utm_source='.$source.'&utm_medium='.$medium.'&utm_term='.$term.'&utm_campaign='.$campaign.'');break;
     }
  }
}
?>