private function getCacheFiles()
    {
        if (!($files = @scandir($this->directory, 1))) {
            return false;
        }

        $negExtLen = -1 * strlen($this->extension);
        $return = array();
        foreach ($files as $file) {
            if (substr($file, $negExtLen) === $this->extension) {
                $return[] = $this->directory . '/' . $file;
            }
        }
        return $return;
    }