private function getCacheFilename($api, $class, $version)
    {
        if (!is_numeric(substr($version, 0, 1))) {
            $versionFile = $version . '.php';
        } else {
            $versionFile = 'v' . $version . '.php';
        }

        // combine the paths and look for the cache file
        $cacheFile = $this->getCacheFolder($api, $class) . $versionFile;

        return $cacheFile;
    }