How to Use PHP usort() for Custom Array Sorting

PHP is a widely used programming language that provides many built‑in functions for developers, among which usort() is used to sort an array according to a user‑defined comparison function.

The syntax of usort() is:

usort(array $array, callable $cmp_function): bool

Here $array is the array to be sorted and $cmp_function is a callable that determines the ordering. The callback must return 0, -1, or 1 to indicate equality, less‑than, or greater‑than respectively.

Comparison function

The comparison function must be callable. It can be an anonymous function or a named function. For example:

function cmp($a, $b) {
    if ($a == $b) {
        return 0;
    }
    return ($a < $b) ? -1 : 1;
}

In this function, $a and $b are the two elements being compared. Returning 0 means they are equal, -1 means $a is smaller, and 1 means $a is larger.

Usage example

Sorting a simple numeric array:

$array = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3];

usort($array, "cmp");

After execution, $array becomes [1, 1, 2, 3, 3, 4, 5, 5, 6, 9].

Sorting an array of associative arrays (e.g., students) by age:

$students = [
    ["name" => "Alice",   "age" => 18],
    ["name" => "Bob",     "age" => 20],
    ["name" => "Charlie", "age" => 19],
];

function cmp_age($a, $b) {
    if ($a["age"] == $b["age"]) {
        return 0;
    }
    return ($a["age"] < $b["age"]) ? -1 : 1;
}

usort($students, "cmp_age");

The resulting order is Alice (18), Charlie (19), Bob (20).

Important notes

The comparison function must be callable.

It must return only 0, -1, or 1. usort() modifies the original array in place, so be aware of side effects.

If the array contains equal elements, usort() does not guarantee their relative order.

Conclusion

The usort() function is a common and powerful tool in PHP for custom sorting of arrays. By defining an appropriate comparison callback and passing it to usort(), developers can sort numbers, strings, objects, or any complex structures while being mindful of the function’s constraints.