public function combine($sources, $options = array())
    {
        $tmpCache = $this->cache;
        $this->cache = new Minify_Cache_Null();

        $env = new Minify_Env();
        $sourceFactory = new Minify_Source_Factory($env, array(
            'checkAllowDirs' => false,
        ), $this->cache);
        $controller = new Minify_Controller_Files($env, $sourceFactory, $this->logger);

        $options = array_merge($options, array(
            'files' => (array)$sources,
            'quiet' => true,
            'encodeMethod' => '',
            'lastModifiedTime' => 0,
        ));
        $out = $this->serve($controller, $options);

        $this->cache = $tmpCache;

        return $out['content'];
    }