private function buildUrl($url)
    {
        $url = $this->protocol . '://' . $url . '/';

        if ($this->date) {
            $url .= $this->date;
        } else {
            $url .= 'latest';
        }

        $url .= '?base=' . $this->base;

        if ($this->key) {
            $url .= '&access_key=' . $this->key;
        }

        if ($symbols = $this->symbols) {
            $url .= '&symbols=' . implode(',', $symbols);
        }

        return $url;
    }