How should I sort this array by key with usort?

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开发者_Go百科 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!

Your items in the array are objects, not associative arrays, so you need to refer to them like this:

function cmp($a, $b) {
  return strcmp($a->slug, $b->slug);
}
usort($allPages, 'cmp')

Your dump of the array says that the elements are Page Objects, not arrays. By chance, do you need to say $a->created_on instead of $a['created_on']? Using object notation instead of array notation.

Just guessing...

As @Tesserex suggests, you need to use object notation instead of array notation.

If you had notices turned on, you'd get error messages about accessing an object as an array.

Another thing to consider, is, your Pages don't all have a 'created_on' attribute, some have a 'published_on' attribute. You'll have to do some error checking / logic inside your usort method to make sure that the key you want to sort by is available, and do something when it's not.