$recaptcha_response = $_POST['g-recaptcha-response'];
$secret_key = '6Lfnl5MnAAAAAJQlMVdmygXQRbsIHnIkmMLMHhHQ';
$response = file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=".$secret_key."&response=".$recaptcha_response);
$response_data = json_decode($response);
if(!$response_data->success){
// handle error
}