Naturally sort an array by alphanumeric key

How can I sort a array like this by its keys, from the smaller resolution to the larger one:

$sizes = array(
   '120x120' => 'large',
   '60x60' => 'small',
   '200x200' => 'very large',
   '90x90' => 'medium',
...

?

should be:

• 60x60
• 90x90
• 120x120
• 200x200
• ...

ksort() in numeric mode should work just fine :

$sizes = array(
   '120x120' => 'large',
   '60x60' => 'small',
   '200x200' => 'very large',
   '90x90' => 'medium',
);

ksort($sizes, SORT_NUMERIC);
var_dump($sizes);

will get you :

array
  '60x60' => string 'small' (length=5)
  '90x90' => string 'medium' (length=6)
  '120x120' => string 'large' (length=5)
  '200x200' => string 'very large' (length=10)

This will work because the size is a numeric -- and is found before the `'x'` *(not sure what will be done with the `'x'` and what follows -- but, anyway, that part of the keys is useless, as it's purely redondant information)*