$cachefile = 'cache.html';
$cachetime = 4 * 60;

if (file_exists($cachefile) && time() - $cachetime < filemtime($cachefile)) {
    include($cachefile);
        echo "<!-- Cached copy, generated ".date('H:i', filemtime($cachefile))." -->\n";
    exit;
}
ob_start();
    /* Heres where you put your page content */
    $cached = fopen($cacheFile, 'w');
    fwrite($cached, ob_get_contents());
    fclose($cached);
ob_end_flush();