Set your Postback URL (on Mu-Online.Org):

Below is code example how to check votes on your side

<?php
                                
    // check if callback is from topsite
    if(!in_array($_SERVER['REMOTE_ADDR'], array('158.69.211.110')))
    {
        die('You are not authorised to access this file.');
    }
    else
    {
        if(isset($_POST['mod']) && $_POST['mod'] == 'reward')
        {
            // remember to escape $_POST variables
            $user = isset($_POST['user']) ? $_POST['user'] : '';
            $site_id = isset($_POST['site_id']) ? (int)$_POST['site_id'] : '';

            /*
             *    and here goes your reward script
             *  connect db
             *  reward user
             */            

             // return response back to topsite

             echo 'OK'; // response good vote was validated
             echo 'VOTED'; // user have already voted
        }
    }
?>