Custom Array Sorting in PHP with usort

Sorting arrays in PHP is a common task, but standard sorting functions like sort() or asort() are often insufficient for complex data structures. This article explains how to use PHP’s usort() function to perform custom array sorting using user-defined comparison functions. You will learn the syntax of usort(), how the spaceship operator (<=>) simplifies comparison logic, and how to apply these concepts to sort multidimensional arrays and objects.

Understanding the usort() Function

The usort() function sorts an array by its values using a user-defined comparison function. It modifies the original array directly (sorting in-place) and assigns new numeric keys to the elements, discarding any existing keys.

The syntax for usort() is:

usort(array &$array, callable $callback): bool

The callback function must accept two parameters (commonly referred to as $a and $b) and return an integer: * Less than 0 (usually -1) if $a should come before $b. * Greater than 0 (usually 1) if $a should come after $b. * Zero (0) if $a and $b are considered equal.

Using the Spaceship Operator (<=>)

Introduced in PHP 7, the spaceship operator (<=>) is the most efficient way to write comparison callbacks. It automatically returns -1, 0, or 1 based on the comparison of two values.

• $a <=> $b returns -1 if $a < $b
• $a <=> $b returns 0 if $a == $b
• $a <=> $b returns 1 if $a > $b

Example 1: Sorting a Simple Array by String Length

In this example, we sort an array of strings from shortest to longest.

<?php
$languages = ["JavaScript", "PHP", "Python", "C", "Go"];

usort($languages, function($a, $b) {
    return strlen($a) <=> strlen($b);
});

print_r($languages);
?>

Output:

Array
(
    [0] => C
    [1] => Go
    [2] => PHP
    [3] => Python
    [4] => JavaScript
)

To sort in descending order (longest to shortest), simply swap the variables in the comparison: strlen($b) <=> strlen($a).

Example 2: Sorting an Array of Associative Arrays

A common real-world scenario is sorting a list of database records or API results. Here, we sort an array of users by their age in ascending order.

<?php
$users = [
    ["name" => "Alice", "age" => 34],
    ["name" => "Bob", "age" => 22],
    ["name" => "Charlie", "age" => 28]
];

// Using an arrow function (PHP 7.4+) for cleaner syntax
usort($users, fn($a, $b) => $a['age'] <=> $b['age']);

print_r($users);
?>

Output:

Array
(
    [0] => Array ([name] => Bob, [age] => 22)
    [1] => Array ([name] => Charlie, [age] => 28)
    [2] => Array ([name] => Alice, [age] => 34)
)

Example 3: Sorting Objects

usort() works identically when sorting arrays of objects. Instead of array keys, you access the object properties.

<?php
class Product {
    public string $name;
    public float $price;

    public function __construct(string $name, float $price) {
        $this->name = $name;
        $this->price = $price;
    }
}

$products = [
    new Product("Laptop", 999.99),
    new Product("Mouse", 25.50),
    new Product("Keyboard", 75.00)
];

// Sort products by price, cheapest to most expensive
usort($products, fn($a, $b) => $a->price <=> $b->price);

print_r($products);
?>