A function is a reusable block of code that, given an input, performs some actions and, optionally, returns some result. You already know several predefined functions like empty, in_array, or var_dump. Those functions come with PHP so you do not have to reinvent the wheel, but you can create your own very easily. You can define functions when you identify portions of your application that have to be executed several times, or just to encapsulate some functionality.
Declaring a function means writing it down so it can be used later. A function has a name, takes some arguments, and has a block of code. Optionally, it can define what kind of value is to be returned. The name of the function has to follow the same rules as variable names, that is, it has to start with a letter or an underscore, and can contain any letters, numbers, or underscore. It cannot be a reserved word.
Let's see a simple example:
function addNumbers($a, $b) {
$sum = $a + $b;
return ...