Horde_Form_Type_enum::isValid PHP Method

    function isValid(&$var, &$vars, $value, &$message)
    {
        if ($var->isRequired() && $value == '' && !isset($this->_values[$value])) {
            $message = Horde_Form_Translation::t("This field is required.");
            return false;
        }
        if (count($this->_values) == 0 || isset($this->_values[$value]) || $this->_prompt && empty($value)) {
            return true;
        }
        $message = Horde_Form_Translation::t("Invalid data submitted.");
        return false;
    }