Functional PHP: A First Approach

Functional PHP? Well, PHP is not a functional language but some functional techniques may be used to improve our code: better readability, easier to maintain => cheaper code. 
For many years, PHP was scripted in a procedural way, all in one file with functions everywhere. After version 5.*, applications have been written using the Object Oriented paradigm (OO). But rarely do we think of PHP in the functional paradigm. Of course, PHP is not a functional language, but we should be able to use the best of each paradigm. It’s not about OO against functional programming (FP), or to define which one is better.