Affiliate links not redirecting please help.

Hi,
I have the following site that I was sent the files for-
reverse-phone-411.com

the affiliate links dont redirect to the clickbank sales pages like they should.
here is a copy of what it says in the go.php file which is what I think it uses for the linking-

<?php

$path = array(
'ReverseGenie' => 'http://818a9zv9p8irfn1bkzrjehcm3o.hop.clickbank.net/?tid=',
'ReversePhoneDetective' => 'http://2c5da2r8hbk-jx6conz2u6i99h.hop.clickbank.net/?tid=',
'UsaPhoneLookup' => 'http://a4b56a-johdyko4lu1nci1rbdx.hop.clickbank.net/?tid='
);

if (array_key_exists($_GET['id'], $path))
header('Location: ' . $path[$_GET['id']] . $_GET['a']);

?>


?>


Is there part of the code missing? or maybe some other problem?
Any advice would be appreciated.
Thanks.