In which file should I define application-wide constants that are specific to my cakephp app?
I define them in app/config/bootstrap.php
Bootstrapping CakePHP
If you have any additional configuration needs, use CakePHP’s bootstrap file, found in app/Config/bootstrap.php. This file is executed just after CakePHP’s core bootstrapping.
This file is ideal for a number of common bootstrapping tasks:
Complete manual goes here