If-Modified-Since
From DreamHost
By querying the If-Modified-Since header, it is possible to build a condition that will return a 304 (Not modified) if the content has not been modified since the client last retrieved a resource. For example, it is possible to use PHP to perform a Conditional GET by examining the value of $_SERVER['HTTP_IF_MODIFIED_SINCE'] and comparing it with a timestamp of the last time the page was modified.
