$servers = array('s1','s2','s3'); // list of subdomains

$ip = $_SERVER['HTTP_CF_CONNECTING_IP']?:$_SERVER['REMOTE_ADDR'];
$i = ((int)str_replace('.','',$ip))%count($servers);
$server = $servers[$i];
$server_current = array_shift((explode('.',$_SERVER['HTTP_HOST'])));
if ($server_current!=$server)
	/**/echo 'Wrong server!'; // replace this by a redirect to the correct location