Thi script redirect not unauthorized users, for example during maintenance.Put ip address of authorized users inside the $admin array.

redirect.php

<?php
//admins address
$admins = array('97.23.95.1', '192.168.1.74');
//redirect page for unauthorized users
$page = 'http://www.sitename.com/redirectpage.html';
 
//////////////////////////////////////////////////////////////////////////
//user ip address
$user = $_SERVER['REMOTE_ADDR'];
//user will be redirect to $page if its ip isn't in the $admin array
//var_dump(array_search($user, $admin));
if(array_search($user, $admins) === FALSE) 
{
  header('Location: ' . $page);
}
?>