php - getting Use of undefined constant admin - assumed 'admin' in php

Question

I am getting the following error while trying to edit the user

Notice: Use of undefined constant admin - assumed 'admin' in /include/user_manager.inc.php on line 902

In line 902 its is like this

if ($user_type === admin) {
                    // Agents cannot edit Admin account
                    return $lang['user_manager_permission_denied'];

Someone please suggest me why this error