How to "include" a function in a view

I've made some code to manipulate a PHP output field.

It's lengthy and I'd like to use it in other views, so, for maintenance reasons, I'd prefer to have only one copy of the code and reference to it from inside the view itself, rather that have multiple versions of the same code in each view.

What would be considered best practise in refactoring this code and getting it referenced from the outside of the view?

i.e. is there a protocol for creating files for this and where they would be located? Would it be the sort of thing I should learn how to make a module for?

Cheers
Duncan