Using PHP usort() Function to Sort Arrays with Custom Comparison Functions

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:

<code>usort(array $array, callable $cmp_function): bool</code>

Here $array is the array to be sorted and $cmp_function is a callable that compares two elements and returns 0, -1, or 1.

Comparison Function

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

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

The function receives two values ( $a and $b ) from the array and should return 0 if they are equal, -1 if $a is smaller, and 1 if $a is larger.

Usage

To sort a numeric array:

<code>$array = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3];
usort($array, "cmp");
// Result: [1, 1, 2, 3, 3, 4, 5, 5, 6, 9]</code>

The same function can sort strings, objects, or associative arrays. For example, sorting an array of student records by age:

<code>$students = [
    ["name" => "Alice",   "age" => 18],
    ["name" => "Bob",     "age" => 20],
    ["name" => "Charlie", "age" => 19],
];
function cmp_age($a, $b) {
    if ($a["age"] == $b["age"]) {
        return 0;
    }
    return ($a["age"] < $b["age"]) ? -1 : 1;
}
usort($students, "cmp_age");
// Result: sorted by age ascending
</code>

Precautions

The comparison function must be callable.

It must return only 0, -1, or 1.

usort() modifies the original array in place.

If multiple elements are equal, their relative order is not guaranteed.

Summary

The usort() function is a common PHP utility for sorting arrays with custom logic; by defining an appropriate comparison callback and passing it to usort() , developers can sort numbers, strings, or complex structures efficiently, while being mindful of the function’s mutability and stability characteristics.