I have read as widely about `usort` as I can, but I cannot 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`):

```php
	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 SO usort question](https://stackoverflow.com/questions/3072445/sorting-by-a-specific-key-of-a-multi-dimensional-array-php/3072489#3072489) -- I just get a blank. Example:

```php
    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!