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usort
Sort an array by values using a user-defined comparison function
Description
true usort(array &$array, callable $callback)
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.
int callback(mixed $a, mixed $b)
Caution
Returning non-integer values from the comparison
function, such as float, will result in an internal cast to
int of the callback's return value. So values such as
0.99 and 0.1 will both be cast to an
integer value of 0, which will compare such values as equal.
Return Values
Always returns true.
Examples
Example #1 usort example
<?php
function cmp($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: $value\n";
}
?>
The above example will output:
The spaceship operator may be used to
simplify the internal comparison even further.
<?php
function cmp($a, $b)
{
return $a <=> $b;
}
$a = array(3, 2, 5, 6, 1);
usort($a, "cmp");
foreach ($a as $key => $value) {
echo "$key: $value\n";
}
?>
Note:
Obviously in this trivial case the sort
function would be more appropriate.
Example #2
usort example using multi-dimensional array
<?php
function cmp($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
<?php
class 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:
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:
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
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