$modx->sendForward(1); 

<?php
$redirect = 'http://Boplo.ir'; // An external URL
// OR
$redirect = $modx->makeUrl(10); // An internal url to document with ID 10

if($modx->getLoginUserID() != 4) { // If current web user's ID is NOT 4
  $modx->sendRedirect($redirect);
}

<?php
$modx->sendForward(1); // Go to Home page

<?php
$modx->config['site_unavailable_page'] // ID of Unavailable page
$modx->config['error_page'] // ID of 404 page