Help me sort this php array using usort()

Question

I have a data structure that looks like

Array
(
[0] => Array
    (
        [0] => something
        [1] => 1296986500
    )

[1] => Array
    (
        [0] => something else
        [1] => 1296600100
    )

[2] => Array
    (
        [0] => another thing
        [1] => 1296831265
    )
)

I'm trying to sort the array based off of the integer which is a unix timestamp. The following function looks right to me but is not sorting the way I want.

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

NOTE when calling this function within a class the OO syntax is the following

uasort($_data, array($this, 'cmp'));