Define/include "globally available" arrays and functions

Instead of creating a new helper file, I could also use app/Common.php, right? It says it's like the Master helper file and loaded already anyway. Any disadvantage of using this file?

Yes absolutely, I forgot about Common.php because I never had to use it yet. The only disadvantage I can think of is if you have a lot of functions and it becomes a mess of thousands of line. Then it would make more sense to have separate helpers for each subject.