No Apache com mod_rewrite crie um arquivo .htaccess
na pasta DocumentRoot
contendo:
Options +FollowSymlinks
RewriteEngine on
RewriteCond %{REQUEST_URI} !/images/maintenance.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]