code: protected function getQueryParam($name, $defaultValue = null)
      {
          if (($params = $this->params) === null) {
              $request = Yii::$app->getRequest();
              $params = $request instanceof Request ? $request->getQueryParams() : [];
          }
  
          return isset($params[$name]) && is_scalar($params[$name]) ? $params[$name] : $defaultValue;
      }
docs: Returns the value of the specified query parameter.
  This method returns the named parameter value from [[params]]. Null is returned if the value does not exist.
  @param string $name the parameter name
  @param string $defaultValue the value to be returned when the specified parameter does not exist in [[params]].
  @return string the parameter value