Master PHP’s usort(): Custom Sorting Made Simple

PHP provides many built‑in functions; usort() sorts an array according to a user‑defined comparison function.

Function signature

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

The first argument is the array to be sorted, the second is a callable that receives two elements and must return 0, -1, or 1.

Comparison function

The comparison function must be callable. A typical example:

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

Returning 0 indicates equality, -1 means the first argument is smaller, and 1 means it is larger.

Basic usage

Sorting a numeric array:

$array = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3];
usort($array, "cmp");

Result:

[1, 1, 2, 3, 3, 4, 5, 5, 6, 9]

Sorting associative arrays

When sorting an array of associative arrays (e.g., students), define a comparator that examines a specific key:

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

After execution the $students array is ordered by the age field from smallest to largest.

Important notes

The comparison function must be callable.

It must return only 0, -1, or 1. usort() sorts the array in place, so the original array is modified.

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

Conclusion

usort()

is a versatile PHP function for custom sorting of arrays, strings, objects, or any data structure where a user‑defined comparator can be applied.