Attempts to cache the sent entity by its
ETag, either supplied or generated
by the hash algorithm specified by the INI setting
If the clients
If-None-Match header matches the supplied/calculated
ETag, the body is considered cached on the clients side and
304 Not Modified status code is issued.
A log entry is written to the cache log if the INI setting
http.log.cache is set
and the cache attempt was successful.
If this function is used outside the http_send_*() API, it facilitates the ob_etaghandler().
FALSE or exits on success with
304 Not Modified if the entity is cached.
See the INI setting
http.force_exit for what "exits" means.