public function get($key, $defaultValue = null)
    {
        $this->open();
        return isset($_SESSION[$key]) ? $_SESSION[$key] : $defaultValue;
    }
public function get($key, $defaultValue = null)
    {
        $this->open();
        return isset($_SESSION[$key]) ? $_SESSION[$key] : $defaultValue;
    }
Returns the session variable value with the session variable name.
If the session variable does not exist, the `$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.