Imagine a world where programmers such as you just enjoy writing code. That world seems enjoyable, perfect, and highly suitable for you in the first place. However, it isn't. What makes a programmer happier and satisfied more than anything in the world is the scenario in which the application reaches the end user. However, if programmers just enjoyed writing their code, who would ensure delivery of the code to the end user?

The end user can be anyone—a naive user, a professional user or even another programmer. So, regardless of the case, scenario, or end user, you, as a programmer, must be careful and concerned to make sure that whatever you code is (re)usable. Throughout your software-engineering life, you were told and you would be told not to reinvent the wheel. If you want this dry principle to be alive, you must make your code usable and reusable.

Another use case! Suppose you are asked to develop an application programming interface (API) in PHP. So, your task at hand is...