Sort an array by values using a user-defined comparison function
Array Functions
PHP Manual

usort

(PHP 4, PHP 5)

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

Description

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

This function will sort an array by its values using a user-supplied comparison function. If the array you wish to sort needs to be sorted by some non-trivial criteria, you should use this function.

Note:

If two members compare as equal, their relative order in the sorted array is 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.

cmp_function

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 integer 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

Returns TRUE on success or FALSE on failure.

Changelog

Version Description
4.1.0 A new sort algorithm was introduced. The cmp_function doesn't keep the original order for elements comparing as equal.

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:

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

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");

while (list($key, $value) = each($fruits)) {
    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 {
    var $name;

    function TestObj($name)
    {
        $this->name = $name;
    }

    /* This is the static comparing function: */
    static function cmp_obj($a, $b)
    {
        $al = strtolower($a->name);
        $bl = strtolower($b->name);
        if ($al == $bl) {
            return 0;
        }
        return ($al > $bl) ? +1 : -1;
    }
}

$a[] = new TestObj("c");
$a[] = new TestObj("b");
$a[] = new TestObj("d");

usort($a, array("TestObj", "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

See Also

Array Functions
PHP Manual
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