$interval=5;
if ($_SERVER['HTTP_IF_MODIFIED_SINCE']){
$ctime = strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE'])+$interval;
if($ctime > time()){
header('HTTP/1.1 304 Not Modified');
exit();
}
}
session_start();
header("Pragma: private");
header("Cache-Control:max-age=$interval, pre-check=$interval");
header("Expires: " . gmdate("D, d M Y H:i:s",time()+$interval)." GMT");
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");
echo date('Y-m-d H:i:s');
echo '
';
print_r($_SERVER['HTTP_IF_MODIFIED_SINCE']);