Master PHP’s usort(): Custom Array Sorting Made Easy

PHP is a widely used programming language that offers many built‑in functions for developers. usort() is one of them, and its main purpose is to sort an array according to a user‑defined comparison function.

Comparison Function

The comparison function passed to usort() must be callable. It is usually defined as 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 represent the two elements being compared. The function should return 0 if they are equal, -1 if $a is smaller, and 1 if $a is larger.

Usage

Below is a basic example of using usort() to sort 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]

usort()

can also sort arrays of strings or objects. For instance, to sort an array of student records 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");
// Result: sorted by age ascending

Important Notes

The comparison function must be callable.

The function must return only 0, -1, or 1. usort() modifies the original array directly, so be aware of side effects.

If multiple elements are equal, usort() does not guarantee their relative order.

Conclusion

The usort() function is a common and powerful tool in PHP development for custom array sorting. By defining an appropriate comparison function and passing it to usort(), developers can sort numbers, strings, or complex objects efficiently, while keeping in mind the noted caveats.