PHP uasort()不是按数组的第三维排序

bool uasort(array &$array, callable $callback)

function compareThirdDimension($a, $b) {
    if ($a[2] == $b[2]) {
        return 0;
    }
    return ($a[2] < $b[2]) ? -1 : 1;
}

$array = array(
    array(1, 2, 3),
    array(4, 5, 6),
    array(7, 8, 9)
);

uasort($array, 'compareThirdDimension');

print_r($array);

Array
(
    [0] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
        )

    [1] => Array
        (
            [0] => 4
            [1] => 5
            [2] => 6
        )

    [2] => Array
        (
            [0] => 7
            [1] => 8
            [2] => 9
        )

)