Zend_Validate_Callback example

$sum += floor($digit / 10) + $digit % 10;
            $weight = $weight % 2 + 1;
        }

        if ((10 - $sum % 10) % 10 != $value[$length - 1]) {
            $this->_error(self::CHECKSUM, $value);
            return false;
        }

        if (!empty($this->_service)) {
            try {
                $callback = new Zend_Validate_Callback($this->_service);
                $callback->setOptions($this->_type);
                if (!$callback->isValid($value)) {
                    $this->_error(self::SERVICE, $value);
                    return false;
                }
            } catch (Zend_Exception $e) {
                $this->_error(self::SERVICEFAILURE, $value);
                return false;
            }
        }