code: public function get($key, $defaultValue = null)
      {
          $this->open();
          return isset($_SESSION[$key]) ? $_SESSION[$key] : $defaultValue;
      }
docs: Returns the session variable value with the session variable name.
  If the session variable does not exist, the `$defaultValue` will be returned.
  @param string $key the session variable name
  @param mixed $defaultValue the default value to be returned when the session variable does not exist.
  @return mixed the session variable value, or $defaultValue if the session variable does not exist.