High Order Functions in PHP: A Practical Guide (6 Examples)

Overview

High order functions are a cornerstone in modern PHP programming, greatly enhancing the way developers write concise and functional code. This guide will explore what high order functions are and how they can be utilized in PHP to simplify your codebase with practical, real-world examples.

Understanding High Order Functions

A high order function is one that either takes a function as an argument, returns a function, or both. PHP offers numerous built-in high order functions that deal with arrays, such as array_map(), array_filter(), and array_reduce(), as well as usort() for sorting.

Example 1: Using array_map()

$numbers = [1, 2, 3, 4, 5];
$squared = array_map(fn($number) => $number ** 2, $numbers);
print_r($squared);

Output:

Array
(
    [0] => 1
    [1] => 4
    [2] => 9
    [3] => 16
    [4] => 25
)

Example 2: Using array_filter()

$numbers = [1, 'two', 3, 'four', 5];
$integers = array_filter($numbers, 'is_int');
print_r($integers);

Output:

Array
(
    [0] => 1
    [2] => 3
    [4] => 5
)

Example 3: Using array_reduce()

$numbers = [1, 2, 3, 4, 5];
$sum = array_reduce($numbers, fn($carry, $item) => $carry + $item, 0);
echo $sum;

Output:

15

Creating Custom High Order Functions

You’re not limited to built-in functions; you can create your own high order functions. Let’s say you want to create a function that applies a custom operation to all elements in an array:

Example 4: Custom High Order Function

function array_apply($array, callable $operation) {
    foreach ($array as $key => $value) {
        $array[$key] = $operation($value);
    }
    return $array;
}

$numbers = [1, 2, 3, 4, 5];
$incremented = array_apply($numbers, fn($n) => $n + 1);
print_r($incremented);

Output:

Array
(
    [0] => 2
    [1] => 3
    [2] => 4
    [3] => 5
    [4] => 6
)

Leveraging Anonymous Functions and Closures

PHP’s support for anonymous functions, also known as closures, makes working with high order functions incredibly straightforward. Closures can capture variables from the parent scope, offering flexibility and power in callbacks.

Example 5: Closure with use Keyword

$multiplier = 3;
$numbers = [1, 2, 3, 4, 5];
$scaled = array_map(fn($n) => $n * $multiplier, $numbers);
print_r($scaled);

Output:

Array
(
    [0] => 3
    [1] => 6
    [2] => 9
    [3] => 12
    [4] => 15
)

Advanced Patterns with High Order Functions

Combining multiple high order functions can lead to powerful data processing patterns. For example, you could combine array_map(), and array_filter() to first transform an array and then filter it.

Example 6: Chaining High the functions together

$numbers = [1, 2, 3, 4, 5];
$oddCubed = array_filter(
    array_map(fn($n) => $n ** 3, $numbers),
    fn($n) => $n % 2 !== 0
);
print_r($oddCubed);

Output:

Array
(
    [0] => 1
    [2] => 27
    [4] => 125
)

Conclusion

High order functions offer a robust framework for operating on collections of data in a functional style in PHP. By understanding and applying them, you can write more readable, expressive, and maintainable code. Embrace these tools to take your PHP coding to the next level.