Global sitewide functions file

hey there

I’m wanting to create a global sitewide functions file and wondering:

a: is it possible

b: how it should look

c: where it should go

I’m wanting to do this so I can reduce the number of plugins I use and take advantage of some snippets instead – hopefully reducing page load time and also cpu load (which is giving me some major grief).

Has anyone done this and can maybe offer a teensy bit of guidance please?