Concept behind using callable variable - php

Question

I have been given a API which I am currently back engineering. there is one function in particular that gives me troubles with fully understanding its purpose/use.
private function split($data, Callable $callback)
{
$split = array();
if ($data) {
$split = array_map(function($joined) use ($callback) {
return $callback(explode('::', $joined));
}, explode(',', $data));
}
return $split;
}
I dont fully understand the concept of Callable, and function within array_map, function($joined) then this function USE callable variable, Could someone explain this concept form me please

Answer 1

A Callable argument is an argument that you can call ! As you can see in the code, the argument $callback is used like a function: $callback(...)
This is called high-order programming and this is really useful in certain cases. A simple example: Let's say you have to code a function that adds 2 and another function that multiplies by 2 every elements of an array. A simple but verbose way to do that is:
function multiply($array) {
$results = array();
foreach ($array as $number) {
$results[] = $number * 2;
}
return $results;
}
function add($array) {
$results = array();
foreach ($array as $number) {
$results[] = $number + 2;
}
return $results;
}
A lot of code is the same in the 2 functions. High-order programming is useful in this case, what you can do is create a function apply($function, $array) that apply the function $function to all the elements of $array and returns an array with the result.
function apply($function, $array) {
$results = array();
foreach ($array as $number) {
$results[] = $function($number);
}
return $results;
}
Now, if you want to multiply all the elements by 2 or add 2, you simply do:
function multiply($array) {
return apply(function($number) {
return $number * 2;
}, $array);
}
function add($array) {
return apply(function($number) {
return $number + 2;
}, $array);
}
You see, we give a function as an argument to the apply function. This function (called $function in apply) is applied to all the elements of $array, and apply returns the results (called $results).
The PHP function array_map does exactly the same thing. When your code calls array_map, it gives a function that takes one argument (the element of the array to process) and returns the processed element (here, it simply applies the function $callback to it).