Make a function that sends a request to your server, in example:
function checkSerial($serial = '1111-1111-1111-1111') {
$server = file_get_contents('http://server.your-domain.com');
if (json_decode($server)) {
$result = json_decode($server);
if ($result[0] == 'valid') return true; else return false
} else return false;
}
and something on server side (index.php):
if (isset($_GET['serial'])) {
if ($_SERVER['REMOTE_ADDR'] == '127.0.0.1' && !empty($_GET['serial']) && $_GET['serial'] == '1111-1111-1111-1111-')) {
$result = array('valid');
} else $result = array('invalid');
} else $result = array('invalid');
echo json_encode($result);
This is just a quick demo how you can implement such a thing!