public function callbackLogs($key, $callback)
    {
        $p = compact('key', 'callback');
        $q = http_build_query($p);

        curl_setopt($this->ch, CURLOPT_POST, false);
        curl_setopt($this->ch, CURLOPT_URL, static::URL.'/callback_log?'.$q);

        if (($resp = curl_exec($this->ch)) === false) {
            throw new HttpError(curl_error($this->ch));
        }

        if (($data = json_decode($resp, true)) === NULL) {
            throw new Error("Unable to decode JSON response from Blockchain: $resp");
        }

        $info = curl_getinfo($this->ch);

        if ($info['http_code'] == 200) {
            return array_map([$this, 'createCallbackLogEntry'], (array) $data);
        }

        throw new Error(implode(', ', $data));
    }