How should I sort this array by key with usort?

Question

I think I might have read every usort article on StackOverflow, but I can't work out this one. It might be that usort is not the tool I need? Here's a bit of the array that I'm working with (I have it assigned to $allPages):

Array
(
    [0] => Page Object
        (
            [id] => 4
            [slug] => articles
            [created_on] => 2009-08-06 07:16:00
        )

    [1] => Page Object
        (
            [id] => 99
            [slug] => a-brief-history
            [created_on] => 2011-04-25 12:07:26
        )

    [2] => Page Object
        (
            [id] => 98
            [slug] => we-arrive
            [created_on] => 2011-04-24 13:52:35
        )

    [3] => Page Object
        (
            [id] => 83
            [slug] => new-year
            [created_on] => 2011-01-02 14:05:12
        )
)

I am trying ultimately to sort on the created_on value, but for the moment, I'd settle on being able to sort on any of them! When I try the normal cmp($a, $b) type callback with usort -- as, for example, in this answer on a usort question -- I just get a blank. Example:

function cmp($a, $b) {
  return strcmp($a["slug"], $b["slug"]);
}
usort($allPages, 'cmp')

And print_r gives me nothing. This is with PHP 5.2.n, not 5.3 btw.

Guidance, please? And thankyou!