Array ( [0] => Array (
                    [card_name] => CardA
                    [str] => 10
                    [def] => 10
                    [desc] => - Recover 150 points of vitality
                              - Attack twice"
                    [qty] => 5
                )
    [1] => Array (
                    [card_name] => CardD
                    [str] => 40
                    [def] => 40
                    [desc] => - Investigate enemy's weakpoint
                    [qty] => 3
                )
    [2] => Array ( [card_name] => CardG
                   [str] => 35
                   [def] => 20
                   [desc] =>
                   [qty] => 1
                )
    [3] => Array (
                   [card_name] => CardH
                   [str] => 25
                   [def] => 30
                   [desc] =>
                   [qty] => 1
                )
    [4] => Array (
                   [card_name] => CardI
                   [str] => 15
                   [def] => 40
                   [desc] => - Enhance strength
                   [qty] => 1
                )
    [5] => Array (
                   [card_name] => CardJ
                   [str] => 5
                   [def] => 50
                   [desc] => - Make silence
                   [qty] => 3
                   )
    )

function compare($valueA, $valueB) {
    if ($valueA['str'] > $valueb['str']) {
        return 1;
    } else if ($valueA['str'] < $valueb['str']) {
        return -1;
    }
    return 0;
}

usort($yourArray, "compare");

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

$a = yourArray[];

usort($a, "cmp");

<?php
    function build_sorter($key) {
        return function ($a, $b) use ($key) {
            return strnatcmp($a[$key], $b[$key]);
        };
    }

    usort($array, build_sorter('def'));
    print_r($array);
?>

foreach($originalArray as $key => $record) {
    $keyValuesArray[$key] = $record['str'];
}

array_multisort($keyvaluesArray, SORT_ASC, $originalArray);