As Johnathan Suggested, you would either want to save it in a cookie or a session.
The easier way would be to use a Session variable.
session_start();
if(!isset($_SESSION['org_referer']))
{
$_SESSION['org_referer'] = $_SERVER['HTTP_REFERER'];
}
Put that at the top of the page, and you will always be able to access the first referer that the site visitor was directed by.