12.0. Introduction

Like all those possessing a library, Aurelian was aware that he was guilty of not knowing his in its entirety.

—Jorge Luis Borges, The Theologians

Imagine that you have two separate programs, both of which work fine by themselves, and you decide to make a third program that combines the best features from the first two. You copy both programs into a new file or cut and paste selected pieces. You find that the two programs had variables and functions with the same names that should remain separate. For example, both might have an init function or a global $count variable. When merged into one program, these separate parts would interfere with each other.