Sort an array by values using a user-defined comparison function

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- Sort an array by values using a user-defined comparison function

# usort

(PHP 4, PHP 5, PHP 7, PHP 8)

usort — Sort an array by values using a user-defined comparison function

### Description

usort(array `&$array`, callable `$callback`): true

Sorts `array` in place by values using a user-supplied comparison function to determine the order.

> Note:

> If two members compare as equal, they retain their original order. Prior to PHP 8.0.0, their relative order in the sorted array was undefined.

> Note: This function assigns new keys to the elements in `array`. It will remove any existing keys that may have been assigned, rather than just reordering the keys.

### Parameters

`array`

  The input array.

`callback`

  The comparison function must return an integer less than, equal to, or greater than zero if the first argument is considered to be respectively less than, equal to, or greater than the second.

  callback(mixed `$a`, mixed `$b`): int
  Caution

  Returning non-integer values from the comparison function, such as float, will result in an internal cast to int of the callback's return value. So values such as `0.99` and `0.1` will both be cast to an integer value of `0`, which will compare such values as equal.

### Return Values

Always returns `true`.

### Changelog

Version | Description
--- | ---
8.2.0 | The return type is `true` now; previously, it was bool.
8.0.0 | If `callback` expects a parameter to be passed by reference, this function will now emit an `E_WARNING`.

### Examples

Example #1 usort() example

    <?phpfunction cmp($a, $b){    if ($a == $b) {        return 0;    }    return ($a < $b) ? -1 : 1;}$a = array(3, 2, 5, 6, 1);usort($a, "cmp");foreach ($a as $key => $value) {    echo "$key: $value\n";}?>

The above example will output:

    0: 1
    1: 2
    2: 3
    3: 5
    4: 6

The spaceship operator may be used to simplify the internal comparison even further.

    <?phpfunction cmp($a, $b){    return $a <=> $b;}$a = array(3, 2, 5, 6, 1);usort($a, "cmp");foreach ($a as $key => $value) {    echo "$key: $value\n";}?>

> Note:

> Obviously in this trivial case the sort() function would be more appropriate.

Example #2 usort() example using multi-dimensional array

    <?phpfunction cmp($a, $b){    return strcmp($a["fruit"], $b["fruit"]);}$fruits[0]["fruit"] = "lemons";$fruits[1]["fruit"] = "apples";$fruits[2]["fruit"] = "grapes";usort($fruits, "cmp");foreach ($fruits as $key => $value) {    echo "\$fruits[$key]: " . $value["fruit"] . "\n";}?>

When sorting a multi-dimensional array, $a and $b contain references to the first index of the array.

The above example will output:

    $fruits[0]: apples
    $fruits[1]: grapes
    $fruits[2]: lemons

Example #3 usort() example using a member function of an object

    <?phpclass TestObj {    private string $name;    function __construct($name)    {        $this->name = $name;    }    /* This is the static comparing function: */    static function cmp_obj($a, $b)    {        return strtolower($a->name) <=> strtolower($b->name);    }}$a[] = new TestObj("c");$a[] = new TestObj("b");$a[] = new TestObj("d");usort($a, [TestObj::class, "cmp_obj"]);foreach ($a as $item) {    echo $item->name . "\n";}?>

The above example will output:

    b
    c
    d

Example #4 usort() example using a closure to sort a multi-dimensional array

    <?php$array[0] = array('key_a' => 'z', 'key_b' => 'c');$array[1] = array('key_a' => 'x', 'key_b' => 'b');$array[2] = array('key_a' => 'y', 'key_b' => 'a');function build_sorter($key) {    return function ($a, $b) use ($key) {        return strnatcmp($a[$key], $b[$key]);    };}usort($array, build_sorter('key_b'));foreach ($array as $item) {    echo $item['key_a'] . ', ' . $item['key_b'] . "\n";}?>

The above example will output:

    y, a
    x, b
    z, c

Example #5 usort() example using the spaceship operator

The spaceship operator allows for straightforward comparison of compound values across multiple axes. The following example will sort `$people` by last name, then by first name if the last name matches.

    <?php$people[0] = ['first' => 'Adam', 'last' => 'West'];$people[1] = ['first' => 'Alec', 'last' => 'Baldwin'];$people[2] = ['first' => 'Adam', 'last' => 'Baldwin'];function sorter(array $a, array $b) {    return [$a['last'], $a['first']] <=> [$b['last'], $b['first']];}usort($people, 'sorter');foreach ($people as $person) {    print $person['last'] . ', ' . $person['first'] . PHP_EOL;}?>

The above example will output:

    Baldwin, Adam
    Baldwin, Alec
    West, Adam

### See Also

- uasort() - Sort an array with a user-defined comparison function and maintain index association
- uksort() - Sort an array by keys using a user-defined comparison function
- The comparison of array sorting functions