code: protected function registerSessionHandler()
      {
          if ($this->handler !== null) {
              if (!is_object($this->handler)) {
                  $this->handler = Yii::createObject($this->handler);
              }
              if (!$this->handler instanceof \SessionHandlerInterface) {
                  throw new InvalidConfigException('"' . get_class($this) . '::handler" must implement the SessionHandlerInterface.');
              }
              @session_set_save_handler($this->handler, false);
          } elseif ($this->getUseCustomStorage()) {
              @session_set_save_handler(
                  [$this, 'openSession'],
                  [$this, 'closeSession'],
                  [$this, 'readSession'],
                  [$this, 'writeSession'],
                  [$this, 'destroySession'],
                  [$this, 'gcSession']
              );
          }
      }
docs: Registers session handler.
  @throws \yii\base\InvalidConfigException