What is the correct HTTP status code to send when a site is down for maintenance?

Solution 1:

HTTP 503 - Service Unavailable would be the most appropriate.

The Web server (running the Web site) is currently unable to handle the HTTP request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay.

This post on the Google Webmaster Central Forum is also relevant:

• Can I restrict Google from crawling my site on a specific day of the week?

Yes - a 503 is the correct server response for "We're closed". If you substitute a normal HTML page saying "We're closed" and serve a 200 it's very likely to get indexed by Google.

If you give the Googlebot a 503, it will just go away and come back later without indexing what you give it.

UPDATE:

As for the php implementation, you may be interested in checking out the following article:

• The holding page and the 503 status code

Also notice the retry-after header, where you can specify after how many seconds another request should be re-attempted.

Solution 2:

From http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=40132

503 (Service unavailable) The server is currently unavailable (because it is overloaded or down for maintenance). Generally, this is a temporary state.

Solution 3:

I'd consider the 503 to be most appropriate, since the 5xx-group is meant for server errors in general.

Solution 4:

503 Service Unavailable