code: private function buildRequestBody(array $request)
      {
          $params = array_reduce(array_keys($request), function ($carry, $key) use($request) {
              $value = $request[$key];
              $carry[$key] = is_array($value) ? implode(' ', $value) : $value;
              return $carry;
          }, []);
  
          return empty($request) ? '' : http_build_query($params, null, '&', PHP_QUERY_RFC3986);
      }
docs: Builds the request body string.
  
  @param array $request Associative array that is the request body.
  
  @return string The request body in URL-encoded format.