I´ve wrote a bit before about PHP and SQL output caching but it was based around caching it within the server/network of the host. This is good thing for the project I was considering it for since it was necessary for the pages that change regularly.
There are, however, caching servers (such as Squid) that take copies of pages to save bandwidth and server load. These servers are generally set up to ignore .php pages since they are generally dynamic by nature, which is a good thing.
It did occure to me that I could reduce server load on some sites that don´t change that often by getting these servers to cache the sites. All I would need to do is send the content type header as html instead of php and the servers should keep a copy. It could be done through the server configuration or something like
header(´Content-type text/html´);
should do it. Only thinking out load really, might give it a try and see what happens.