private function performRequest($command, $parameters)
        {
        // add the apikey to the parameters
        $parameters["key"] = $this->api_key;

        // make an array out of the dictionary so that each element is a key value pair glued together with an '=', and urlencode the parameters
        $param_array = array();
        foreach ($parameters as $key=>$value)
            $param_array[] = "$key=" . urlencode($value);

        // glue the array together unto a string with elements connected with '&'
        $params = implode("&", $param_array);

        // put the whole URL together now
        $url = $this->base_url . $command . "?" . $params;

        // cal the server
        $json = $this->call($url);

        $data = json_decode($json, true);

        if (isset($data["error"]))
          {
          $this->error["code"]    = $data["error"]["code"];
          $this->error["message"] = $data["error"]["message"];
          $data = false;
          }

        return $data;
        }