(PECL pecl_http >= 0.1.0)
http_cache_last_modified — Caching by last modification
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.
Example #1 A http_cache_last_modified() example
Caching for 5 seconds.