protected function getPersistentContent()
    {
        if (!$this->persistent || null === ($value = $this->getValue())) {
            return '';
        }
        return '<input type="hidden"' . self::getAttributesString(array(
            'name'  => $this->getName(),
            'value' => $value,
            'id'    => $this->getId()
        )) . ' />';
    }