Getting referral url in php?

[well well well]

Question for you experts. How can I get and send the referral url in the redirection script below? I posted similar question before but the person who answered included the wrong info. ($link) for getting and mailing the referral url in the body of the email which is what I need now. Again, I need to get the referral url of the page the user clicked the link on not the target url.


<?PHP

/* This script name is jump.php

<a href="jump.php?m=XXXXXX">XXXXXX</a>
XXXXXX is the target link.
*/


$m = $_GET['m'];

if ($m == "") {$link = "http://www.externalsite.com/library.htm… // Default Blank
if ($m == "Abraham-L") {$link = "http://www.externalsite.com/abraham-lin…
if ($m == "Washington") {$link = "http://www.externalsite.com/washington.…

if(false == mail("[email protected]", $m, $link)) echo 'Sending email for '.$m.' to '.$link.' failed.';

header("Location: $link");
?>
James, where is the referral url?