code: public function credit(): ?array
      {
          $this->checkConfig();
  
          $query = http_build_query([
              'userkey' => $this->userkey,
              'passkey' => $this->passkey,
          ]);
  
          $response = Request::get($this->baseUrl.'/smsapibalance.php?'.$query);
  
          $xml = simplexml_load_string($response->body);
          $body = json_decode(json_encode($xml), true);
  
          return [
              'code'    => $response->code,
              'message' => ($response->code == 200) ? 'OK' : $body['message']['text'] ?? '',
              'data'    => $body,
          ];
      }
docs: Check credit balance.
  
  @return array|null