$con_id = connect_db();
$query = "UPDATE Visitors SET Acknow = -1 WHERE Person_ID = ? AND Visit_ID = ?";
$sql_params = array (
    (int)$_POST['pers_nr'],
    (int)$_POST['bes_id'],
    );
$result = sqlsrv_query($con_id, $query, $sql_params);