private function change_membership($user_id, $membership_id)
{
/** changing membership of the user **/
wpl_users::change_membership($user_id, $membership_id);
$res = 1;
$message = $res ? __('Operation was successful.', WPL_TEXTDOMAIN) : __('Error Occured.', WPL_TEXTDOMAIN);
$data = NULL;
$response = array('success' => $res, 'message' => $message, 'data' => $data);
echo json_encode($response);
exit;
}