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.