code: public function getSandboxSignKey()
      {
          $this->setSign();
  
          $xml = DataParser::toXml($this->getData());
  
          $url = self::SANDBOX_URL;
  
          $client = new Client([
              'timeout' => '10.0'
          ]);
          $options = [
              'body' => $xml,
              'http_errors' => false
          ];
  
          $response = $client->request('POST', self::SANDBOX_URL, $options);
  
          if ($response->getStatusCode() != '200') {
              throw new PayException('网络发生错误，请稍后再试curl返回码：' . $response->getReasonPhrase());
          }
          // 格式化为数组
          $retData = DataParser::toArray($response->getBody()->getContents());
          if (strtoupper($retData['return_code']) != 'SUCCESS') {
              throw new PayException('微信返回错误提示:' . $retData['retmsg']);
          }
  
          return $retData['sandbox_signkey'];
      }
docs: 获取沙盒的签名信息
  @return mixed
  @throws PayException