Order of loading config and constants

If you define defaults in config/constants.php and it is loaded before config/ENVIRONMENT/constants.php, then you'd always have the defaults ... Constants aren't variables, you can't redefine them.
In order to get the cascading effect that you want, it has to work like it currently does (higher precedence first).

But on a more important side-note - reliance on constants is a design flaw in 99% of the cases; you probably don't need them at all. I can't even remember the last time I declared a global constant.