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

PHP provides the usort() function to sort arrays based on a user‑defined comparison function. The function signature is:

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

The comparison function must be callable and return 0, -1, or 1. A typical example is:

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

Using this, a numeric array can be sorted as follows:

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

For associative arrays, such as a list of students, you can sort by a specific field (e.g., age) with a custom comparator:

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

Key points to remember when using usort():

The comparison function must be callable.

It must return only 0, -1, or 1. usort() modifies the original array in place.

The function does not guarantee stable ordering for equal elements.

In summary, usort() is a versatile PHP function for custom array sorting, requiring a proper comparison callback and awareness of its side effects.