Writing Admin/User/Public controllers to insure authentication

That is why constants were invented!

Not exactly. Constants were invented to contain constant values. Like pi (3.14159265358979323846) or e (2.71828182845904523536).

Constants were invented to contain a value that doesn't change during the execution. It doesn't need to be something like PI ! Anyway all projects are different and not all projects are super complex with tons of other developers.

Also, the argument was:

Personally, I have a lot of controllers, and I don't want to have to change every file which extends Admin_Controller or Front_Controller just because the file containing that class moves to a new location or the name of the file changes for some reason.

...that's why I talked about constants. I never said constants were better than autoloaders!