tag.php
public function setValue($value)
    {
        if ($value === false) {
            return parent::setValue($value);
        }
        if ($value != $this->getAttribute('value')) {
            return false;
        }

        return parent::setValue($value);
    }
public function setValue($value)
    {
        if ($value === false) {
            return parent::setValue($value);
        }
        if ($value != $this->getAttribute('value')) {
            return false;
        }

        return parent::setValue($value);
    }
The only allowed value sn the one in the "value" attribute.

@param string $value      New value.

@return bool           True if allowed.