How do I sort a multi-dimensional array by value?

Question

I have an array as following and I want to order that array by the value of the key "attack". First keys of the arrays (15, 13, 18) are ID of some certain item from database, so I don't want these keys to be changed when the array is sorted. Any help would be greatly appreciated.

This is the array:

$data = array('15' => array('attack' => '45', 'defence' => '15', 'total' => '10'),
          '13' => array('attack' => '25', 'defence' => '15', 'total' => '10'),
          '18' => array('attack' => '35', 'defence' => '15', 'total' => '10')
    );

Answer 1

Use uasort():

This function sorts an array such that array indices maintain their correlation with the array elements they are associated with, using a user-defined comparison function.

This is used mainly when sorting associative arrays where the actual element order is significant.

Example:

function cmp($a, $b) {
    if ($a['attack'] == $b['attack']) {
        return 0;
    }
    return ($a['attack'] < $b['attack']) ? -1 : 1;
} 

uasort($data, 'cmp');

If the values are always strings, you can also use strcmp() in the cmp() function:

function cmp($a, $b) {
    return strcmp($a['attack'], $b['attack']);
} 

Update:

To sort in descending order you just have to change the return values:

return ($a['attack'] < $b['attack']) ? 1 : -1;
//                                     ^----^

or to pick up @salathe's proposal:

return $b['attack'] - $a['attack'];

Answer 2

$data = array('15' => array('attack' => '45', 'defence' => '15', 'total' => '10'),
          '13' => array('attack' => '25', 'defence' => '15', 'total' => '10'),
          '18' => array('attack' => '35', 'defence' => '15', 'total' => '10')
    );

uasort($data, 'cmp');

function cmp($x, $y) {
    if ($x['attack'] > $y['attack']) {
        return 1;
    } elseif ($x['attack'] < $y['attack']) {
        return -1;
    }
    return 0;
}

print_r($data);

PRINTS:

Array
(
    [13] => Array
        (
            [attack] => 25
            [defence] => 15
            [total] => 10
        )

    [18] => Array
        (
            [attack] => 35
            [defence] => 15
            [total] => 10
        )

    [15] => Array
        (
            [attack] => 45
            [defence] => 15
            [total] => 10
        )

)