public function responseHeaderCallback($ch, $header)
    {
        if (strpos($header, ':')) {
            list ($key, $value) = explode(':', $header, 2);

            $value = trim($value);

            if (!empty($value)) {
                if (!isset($this->responseHeaders[$key])) {
                    $this->responseHeaders[$key] = $value;
                } elseif (is_array($this->responseHeaders[$key])) {
                    $this->responseHeaders[$key] = array_merge($this->responseHeaders[$key], [$value]);
                } else {
                    $this->responseHeaders[$key] = array_merge([$this->responseHeaders[$key]], [$value]);
                }
            }
        }

        return strlen($header);
    }