Sort rows in a 2d array by one column while preserving original columnar relationships

My array looks like this:

Array
(
[0] => Array
    (
        [0] => 1
        [1] => 6
        [2] => 4
        [3] => 5
    )

[1] => Array
    (
        [0] => 272.05
        [1] => 63.54
        [2] => 544.79
        [3] => 190.62
    )

[2] => Array
    (
        [0] => 2011-03-06 14:08:19
        [1] => 2011-03-06 14:29:04
        [2] => 2011-03-06 14:28:39
        [3] => 2011-03-06 14:29:28
    )

)

I want to sort by $myArray[1]. I have this usort function:

function sortAmount($a, $b) { 
    return strnatcmp($a[1], $b[1]); 
}

It is called like this:

usort($myArray, "sortAmount"); 

However, the array does not change after calling usort. I want the numbers in $myArray[1] to be sorted in ascending order, and for the corresponding indexes in $myArray[0] and $myArray[2] to change with it.

I think you want array_multisort:

array_multisort($a[1], $a[0], $a[2]);

gives

Array
(
    [0] => Array
        (
            [0] => 6
            [1] => 5
            [2] => 1
            [3] => 4
        )

    [1] => Array
        (
            [0] => 63.54
            [1] => 190.62
            [2] => 272.05
            [3] => 544.79
        )

    [2] => Array
        (
            [0] => 2011-03-06 14:29:04
            [1] => 2011-03-06 14:29:28
            [2] => 2011-03-06 14:08:19
            [3] => 2011-03-06 14:28:39
        )

)

Apart from that, why do you use strcmp to compare numbers?