Writing Admin/User/Public controllers to insure authentication

Code will become less portable, other people will be forced to use your set of constants, they might have already invented their own set (if they use the same approach too). Second, it is useless work to document these constants.

Less portable? A constant allows you to define a path and a filename in a single place. How can this be less portable than hard-coding everywhere?

Also, you will think "this constant has a ugly name, let me deprecate it in favor of that one" - useless work.

It's the same with everything: function, class name, configuration item...