(PHP 4, PHP 5, PHP 7, PHP 8)
usort — Sort an array by values using a user-defined comparison function
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.
arrayThe input array.
callbackThe 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.
Always returns true.
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