public static function getModifiedSinceHeader()
    {
        $modifiedSince = '';
        if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
            $modifiedSince = $_SERVER['HTTP_IF_MODIFIED_SINCE'];

            // strip any trailing data appended to header
            if (false !== ($semicolonPos = strpos($modifiedSince, ';'))) {
                $modifiedSince = substr($modifiedSince, 0, $semicolonPos);
            }
        }
        return $modifiedSince;
    }