This little piece of code checks the last modification time of the script it's
in, generates an ETag based on the full REQUEST_URI + timestamp, and checks to
see if it can send back a 304 Not Modified status instead of continuing on
processing the request further on.
For older browsers, it will also send a Last-Modified header, and check for a
If-Modified-Since header in the request to achieve the same thing...

Note that there are some curious side-effects - for instance, if you're viewing
a page as anonymous visitor and you log in, the (cached) page you're on will
still show you as *not* logged in, while any other part of the site will see
that you *are* logged in :-)

$thisfile = $_SERVER['SCRIPT_FILENAME'];
if (!empty($thisfile) && file_exists($thisfile)) {
    $thistime = filemtime($thisfile);
    $md5 = md5($_SERVER['REQUEST_URI'] . $thistime);
    header("ETag: $md5");
    //global $_SERVER;
    if (!empty($_SERVER['HTTP_IF_NONE_MATCH']) &&
        preg_match("/$md5/", $_SERVER['HTTP_IF_NONE_MATCH'])) {
        header('HTTP/1.0 304');
        // For IIS ?
        //header('Status: 304 Not Modified');
        exit;
    } elseif (!empty($_SERVER['HTTP_IF_MODIFIED_SINCE']) &&
              strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= $thistime) {
        header('HTTP/1.0 304');
        // For IIS ?
        //header('Status: 304 Not Modified');
        exit;
    }
    header("Last-Modified: " . gmdate("D, d M Y H:i:s",$thistime) . " GMT");
    session_cache_limiter('private');
    // For Mozilla ?
    //session_cache_limiter('private_no_expire');
}

For more tips and tricks, see
<a href="http://mikespub.net/copdab/blank.php">Mike's Pub</a>