php - Notice: Use of undefined constant success - assumed 'success'

Anyone able to assist on fixing this one annoying error been trying for like 30minutes.

if(isset($_POST['statusBtn']))
{
    $username = htmlspecialchars($_POST['address']);
    strtolower($username);

    if(empty($username))
    {
        echo '
Fill in all fields';
    } else if($username == "" || $username == ""){
        echo '
Domain Whitelisted';
    } else {
        $geo = @file_get_contents("http://ip-api.com/json/{$username}");
        $json_a = json_decode($geo, true);
        $proxyid = $json_a['status'];

        if($proxyid == success)
        {
            echo '
'.htmlspecialchars($username).' is Online!';
        } else {
            echo '
'.htmlspecialchars($username).' is Offline!';
        }
    }
}
?>

When I submit for example google.com in form it echo's the result along with error

Notice: Use of undefined constant success - assumed 'success'