Let’s start by defining the word “function.” A function is a self-contained piece of code which carries out a particular task (or function!). A key benefit of using functions is that they are reusable; if you have a task that needs to be performed a number of times, a function is an ideal solution. They can be either defined by you or by PHP (PHP has a rich collection of built-in functions). This article will focus on programmer-defined functions but will touch briefly on PHP’s functions to complete the picture.
Link: PHP Master