Landing page redirect script based on referrer? (2)

Any wiz's know of a reliable script to put on your LP that redirects all users to some other page if they don't come from your tracker?

Basically: if user comes from your tracker, script does nothing. if user did not come from your tracker, redirect them to a different page

Not the most secure thing ever (there are various ways to get around it), but if you place this at the very top of a landing page which is implemented in PHP it should work.

$my_tracker_url = "track.mydomain.com"; 
if (strpos($_SERVER['HTTP_REFERER'], $my_tracker_url) == false) 
{
   header('Location: http://www.placetogoifyoudontcomefrommytracker.com/');
}