Cake PHP在一个地方定义String Constants

I am using cake PHP 2.1. I want to show an error message something like 'Invalid Parameter' in many places on my application.I want to define it in one place and use in many places.how can i achive this in Cake PHP.

Simply define a Constant in YourApplication/app/config/bootstrap.php and use that constant in your application

define('INVALID_MSG','Invalid Parameter');