Simply put the code in your sites functions.php and call it in page_head.php!

check_referer()

check_referer("./difname")

function is_ipv4($s)
{
    $n = "([0-9]|1?\d\d|2[0-4]\d|25[0-5])";
    return preg_match("#^{$n}\.{$n}\.{$n}\.{$n}$#", $s);
}
function check_referer($db_referer = "./referer.txt")
{
    if ( !preg_match("#http://(.*?)/(.*)#", $_SERVER["HTTP_REFERER"], $m) )
    {
        return false;
    }
    if ( !is_ipv4($m[1]) && is_numeric(strpos($m[1], '.')) && substr_count($m[1], '.') > 1 )
    {
        $m[1] = strrev($m[1]);
        $m[1] = substr($m[1], 0, strpos($m[1], '.', strpos($m[1], '.') + 1));
        $m[1] = strrev($m[1]);
    }
    if ( $m[1] == $_SERVER["HTTP_HOST"] )
    {
        return false;
    }
    if ( file_exists($db_referer) )
    {
        $referer = unserialize(file_get_contents($db_referer));
    }
    else
    {
        $referer = array();
    }
    if ( !isset($referer[$m[1]]) )
    {
        $referer[$m[1]] = 0;
    }
    $referer[$m[1]]++;
    file_put_contents($db_referer, serialize($referer));
    return true;
}