Maintenance is a project mainly written in ..., it's free.
Force your CakePHP app into maintenance mode
A plugin to activate/deactivate maintenance mode for CakePHP apps.
There's a view file if you need to change the message or appearance.
Load the component in you app_controller.php
var $components = array('Maintenance.Maintenance');This will force your application into maintenance mode, so each request will be redirected to /maintenance.
How if you want the URL like / ? That's easy ... look at maintenance/config/settings.php and read the example or just copy require_once block into APP/config/routes.php and put on top of other URL patterns.
require_once App::pluginPath('maintenance').'/config/routes.php';