( [int timestamp_or_expires] )
Attempts to cache the sent entity by its last modification date.
If the supplied argument is greater than 0, it is handled as timestamp
and will be sent as date of last modification. If it is 0 or omitted,
the current time will be sent as Last-Modified date. If it's negative,
it is handled as expiration time in seconds, which means that if the
requested last modification date is not between the calculated timespan,
the Last-Modified header is updated and the actual body will be sent.
A log entry will be written to the cache log if the
INI setting http.log.cache
is set and the cache attempt was successful.