Site in maintenance. How to redirect visitors?

My PHP site in maintenance. How to redirect visitors to a "site in maintenance" single page?

I heard something about app_offline.htm for ASP.NET.

Is there something similar for PHP?

• I want that every page from "mysite.com" be redirected to "maintenance.php";
• I don't want to do the minimum modification in the existing site pages, ideally no one.

Apache version 2.2.15 under Linux
PHP version 5.2.13

As my Apache in under Linux I do the following:

Created ".htaccess" file under the site root with the folowing content:

Options +FollowSymlinks
RewriteEngine on

RewriteCond %{REQUEST_URI} !/images/mainetnance.png$
RewriteCond %{REQUEST_URI} !/maintenance.php$

# here filter the developer's IP/
#RewriteCond %{REMOTE_HOST} !^888\.888\.888\.888

RewriteRule $ /maintenance.php [R=302,L]

Is to remark that I added to filter the .png files, cause I use an image (/images/mainetnance.png) for the maintenance.php, and this image should remain visible, because of global redirect.