How to use HTTP cache headers with PHP

etag.php

<?php 

// https://stackoverflow.com/a/1973016/3360295

$tsstring = gmdate('D, d M Y H:i:s ', $timestamp) . 'GMT';
$etag = $language . $timestamp;

$if_modified_since = isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? $_SERVER['HTTP_IF_MODIFIED_SINCE'] : false;
$if_none_match = isset($_SERVER['HTTP_IF_NONE_MATCH']) ? $_SERVER['HTTP_IF_NONE_MATCH'] : false;
if ((($if_none_match && $if_none_match == $etag) || (!$if_none_match)) &&
    ($if_modified_since && $if_modified_since == $tsstring))
{
    header('HTTP/1.1 304 Not Modified');
    exit();
}
else
{
    header("Last-Modified: $tsstring");
    header("ETag: \"{$etag}\"");
}