It’s very common for a webmaster to take their site down to perform maintenance. Did you know that as website maintenance is being performed, Google search engine rankings could potentially drop since Googlebot cannot reach your site? Use a 503 HTTP result code to make sure that this doesn’t happen to you.
The 503 HTTP result code will tell Google that your site being down is not permanent. If you use the optional Retry-After header, you can also inform Googlebot when it should attempt to re-visit your site. Make sure to not use this code as an ongoing solution because Google might view your site as being down permanently and you risk it being removed entirely from the Google index.
Here is an example 503 HTTP result code:
header(‘HTTP/1.1 503 Service Temporarily Unavailable’);
header(‘Retry-After: Fri, 1 Jun 2012 12:00:00 EST’);