Why ksort does not work? (PHP)

Question

If I run this :

echo '<pre>';print_r($a);echo '</pre>';

The result :

Array ( [0] => stdClass Object ( [id] => 3 [name] => chelsea.png ) [1] => stdClass Object ( [id] => 4 [name] => arsenal.png ) [2] => stdClass Object ( [id] => 5 [name] => mu.png ) )

If I run this :

echo '<pre>';print_r($b);echo '</pre>';

The result :

Array ( [0] => Array ( [id] => 1 [name] => city.png ) )

Then I merge use :

$c = array_merge($a, $b);

echo '<pre>';print_r($c);echo '</pre>';

The result :

Array ( [0] => stdClass Object ( [id] => 3 [name] => chelsea.png ) [1] => stdClass Object ( [id] => 4 [name] => arsenal.png ) [2] => stdClass Object ( [id] => 5 [name] => mu.png ) [3] => Array ( [id] => 1 [name] => city.png ) )

I want to sort the array by id

I try this :

ksort($c);

Then I run

echo '<pre>';print_r($c);echo '</pre>';

The result :

Array ( [0] => stdClass Object ( [id] => 3 [name] => chelsea.png ) [1] => stdClass Object ( [id] => 4 [name] => arsenal.png ) [2] => stdClass Object ( [id] => 5 [name] => mu.png ) [3] => Array ( [id] => 1 [name] => city.png ) )

It does not work. Position id = 1 has a position at the very end. It should be in the first position

How can I solve it?