PHP Manual: http_send_last_modified
http_send_last_modified
Description
bool http_send_last_modified ( [int timestamp] )Send a Last-Modified header with a valid HTTP date.
Note: This function is supposed to be used in conjunction with http_send_data(), http_send_file() and http_send_stream().
Parameters
timestamp
a Unix timestamp, converted to a valid HTTP date; if omitted, the current time will be sent