<?php
//get the last-modified-date of this very file
$lastModified=filemtime($_SERVER['SCRIPT_FILENAME']);
//get a unique hash of this file (etag)
$etagFile = md5_file($_SERVER['SCRIPT_FILENAME']);
//get the HTTP_IF_MODIFIED_SINCE header if set
$ifModifiedSince=(isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? $_SERVER['HTTP_IF_MODIFIED_SINCE'] : false);
//get the HTTP_IF_NONE_MATCH header if set (etag: unique file hash)
$etagHeader=(isset($_SERVER['HTTP_IF_NONE_MATCH']) ? trim($_SERVER['HTTP_IF_NONE_MATCH']) : false);
header("Last-Modified: ".gmdate("D, d M Y H:i:s", $lastModified)." GMT");
header("Etag: $etagFile");
$ttc=60*60*24*8;
header('Cache-Control: private; max-age='.$ttc);
header('Expires: '.gmdate("D, d M Y H:i:s", time() + $ttc);
header("Pragma: cache");
//check if page has changed. If not, send 304 and exit
if (@strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE'])==$lastModified || $etagHeader == $etagFile)
{
header("HTTP/1.1 304 Not Modified");
exit;
}
//your normal code
echo "<!-- This page was last modified: ".date("d.m.Y H:i:s",$lastModified)." -->";
?>