private function getIfModifiedSinceTimestamp()
    {
        if (!empty($_SERVER[self::HTTP_IF_MODIFIED_SINCE])) {
            $modifiedSinceString = $_SERVER[self::HTTP_IF_MODIFIED_SINCE];
        } elseif (!empty($_ENV[self::HTTP_IF_MODIFIED_SINCE])) {
            $modifiedSinceString = $_ENV[self::HTTP_IF_MODIFIED_SINCE];
        } else {
            $modifiedSinceString = null;
        }

        if ($modifiedSinceString) {
            // Some versions of IE 6 append "; length=##"
            if (($pos = strpos($modifiedSinceString, ';')) !== false) {
                $modifiedSinceString = substr($modifiedSinceString, 0, $pos);
            }

            return strtotime($modifiedSinceString);
        }

        return false;
    }