<?PHP

$Allowed_domains = array(
        'a-good-domain.COM','another-nice-one.org',);

$allowed = false;
foreach ($allowed_domains as $a) {
    if (PReg_match("@https?://$a/.*@",$_SERVER['HTTP_REFERER'])) {
        $allowed = true;
    } 
}

if ($allowed) 
    echo "Nice domain";
else
    echo "Ugly domain";