In PHP, a callback function is a function that can be passed as an argument to another function or used as a variable. Callback functions are commonly used in various scenarios, such as event handling, sorting, filtering, and more.

Here's a simple example to demonstrate the concept of callback functions in PHP:

php
<?php
// Example callback function
function processNumbers($numbers, $callback) {
    $result = [];
    foreach ($numbers as $number) {
        $processedNumber = $callback($number);
        $result[] = $processedNumber;
    }
    return $result;
}

// Callback function to square a number
function square($number) {
    return $number * $number;
}

// Callback function to double a number
function doubleNumber($number) {
    return $number * 2;
}

$numbers = [1, 2, 3, 4, 5];

// Using the processNumbers function with different callback functions
$squaredNumbers = processNumbers($numbers, 'square');
$doubleNumbers = processNumbers($numbers, 'doubleNumber');

print_r($squaredNumbers);
print_r($doubleNumbers);
?>

In this example, we have two callback functions (`square` and `doubleNumber`). The `processNumbers` function takes an array of numbers and a callback function as arguments. It processes each number in the array using the provided callback function and returns an array of processed results.

We then call the `processNumbers` function twice, each time using a different callback function. This demonstrates how callback functions can be passed as arguments and used to customize the behavior of another function.

Note that in modern PHP, you can also use anonymous functions (also known as closures) as callback functions. Here's an example using anonymous functions:

php
$incrementNumbers = processNumbers($numbers, function($number) {
    return $number + 1;
});

print_r($incrementNumbers);

In this example, an anonymous function is defined inline and passed directly as the callback to the `processNumbers` function.

Callbacks are a powerful concept in PHP that enable you to create flexible and reusable code by dynamically changing the behavior of functions based on different requirements.

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