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usort

(3.0.3 - 3.0.18 only, PHP 4 >= 4.0.0)

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

Description

void usort (array array, string 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.

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

Example 1. usort() example

function cmp ($a, $b) {
    if ($a == $b) return 0;
    return ($a > $b) ? -1 : 1;
}

$a = array (3, 2, 5, 6, 1);

usort ($a, "cmp");

while (list ($key, $value) = each ($a)) {
    echo "$key: $value\n";
}

This example would display: 

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

Note: Obviously in this trivial case the rsort() function would be more appropriate.

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

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.

This example would display: 

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

Note: Instead of a function name, an array containing an object reference and a method name can also be supplied.

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

class TestObj {
    var $name;

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

    /* This is the static comparing function: */
    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");

uasort($a, array ("TestObj", "cmp_obj"));

foreach ($a as $item) {
    print $item->name."\n";
}

This example would display: 

b
c
d

Warning

The underlying quicksort function in some C libraries (such as on Solaris systems) may cause PHP to crash if the comparison function does not return consistent values.

See also uasort(), uksort(), sort(), asort(), arsort(),ksort(), natsort(), and rsort().

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