php_http_cache_controll.php

function set_headers($file, $timestamp) {
    $gmt_mtime = gmdate('r', $timestamp);
    header('ETag: "'.md5($timestamp.$file).'"');
    header('Last-Modified: '.$gmt_mtime);
    header('Cache-Control: public');
    if(isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) || isset($_SERVER['HTTP_IF_NONE_MATCH'])) {
        if ($_SERVER['HTTP_IF_MODIFIED_SINCE'] == $gmt_mtime
        || str_replace('"', '', stripslashes($_SERVER['HTTP_IF_NONE_MATCH'])) == md5($timestamp.$file)) {
            header('HTTP/1.1 304 Not Modified');
            exit();
        }
    }
}