What's the use of a callback function in PHP? - php

Question

I'm in the process of learning PHP, and the concept of callback functions have me slightly confused. I understand that there are synchronous and asynchronous callbacks and that the common callback in PHP is synchronous. I have read through a lot of information regarding this topic, but I'm none the wiser still.
How is this:
function string($string, $callback) {
$results = array(
'upper' => strtoupper($string),
'lower' => strtolower($string)
);
if(is_callable($callback)) {
call_user_func($callback, $results);
}
}
string('Danny', function($name) {
echo $name['upper'];
}
);
Different or better than this:
function string($string, $case) {
$options = [
'upper' => strtoupper($string),
'lower' => strtolower($string)
];
echo $options[$case];
}
string('Danny', 'upper');

Answer 1

Here's a typical example where the callback is better than the direct value:
function intricateProcessToCalculateDefault() {
//Takes 3 seconds
}
function valueOrDefault($value, $default = null) {
if ($value === null) {
if (is_callable($default)) {
return $default();
}
return $default;
}
return $value;
}
Now consider these two examples:
$valueToUse = valueOrDefault(
$_GET["parameterWithFallback"] ?? null,
intricateProcessToCalculateDefault()
);
// or
$valueToUse = valueOrDefault(
$_GET["parameterWithFallback"] ?? null,
function () { return intricateProcessToCalculateDefault(); }
);
Now here's the thing. If you don't pass parameterWithCallback this will take 3 seconds either way. However if you do then intricateProcessToCalculateDefault will never be called if passed as a callback and will therefore save you time.
In your case, since the function code is ran in either case, there's no benefit.
PHP is mostly synchronous so there's no easy way to get asynchronous functions. Some libraries do offer them but that's usually tied with calling a shell executable or something similar.
Another use case is for inversion for control. There are many generic algorithms which can be implemented with a "placeholder" for user code for. array_map is such an example.
$array = [ "string1", "sting2" ];
$action = "upper";
array_map(function ($value) use ($action) {
return ucfirst(strtolower($value));
}, $array);
In this case control is inverted. A PHP function is calling your function instead of the other way around.

Answer 2

In this example with a callback you could easily pass a different function without changing the original code:
<?php
function string($string, $callback) {
$results = array(
'upper' => strtoupper($string),
'lower' => strtolower($string)
);
if(is_callable($callback)) {
call_user_func($callback, $results);
}
}
string('Danny', function($name) {
echo $name['upper'];
}
);
string('Danny', function($name) {
echo 'Mr.'.$name['upper'];
}
);
string('Danny', function($name) {
echo 'Mrs.'.$name['upper'];
}
);