Group array by subarray values

I have an array of subarrays in the following format:

array
(
    a => array ( id = 20, name = chimpanzee )
    b => array ( id = 40, name = meeting )
    c => array ( id = 20, name = dynasty )
    d => array ( id = 50, name = chocolate )
    e => array ( id = 10, name = bananas )
    f => array ( id = 50, name = fantasy )
    g => array ( id = 50, name = football )
)

And I would like to group it into a new array based on the id field in each subarray.

array
(
    10 => array
          (
            e => array ( id = 10, name = bananas )
          )
    20 => array
          (
            a => array ( id = 20, name = chimpanzee )
            c => array ( id = 20, name = dynasty )
          )
    40 => array
          (
            b => array ( id = 40, name = meeting )
          )
    50 => array
          (
            d => array ( id = 50, name = chocolate )
            f => array ( id = 50, name = fantasy )
            g => array ( id = 50, name = football )
          )
)

$arr = array();

foreach ($old_arr as $key => $item) {
   $arr[$item['id']][$key] = $item;
}

ksort($arr, SORT_NUMERIC);

foreach($array as $key => $value){
   $newarray[$value['id']][$key] = $value;
}

var_dump($newarray);

piece of cake ;)

The following code adapts @Tim Cooper’s code to mitigate Undefined index: id errors in the event that one of the inner arrays doesn’t contain an id:

$arr = array();

foreach($old_arr as $key => $item)
{
    if(array_key_exists('id', $item))
        $arr[$item['id']][$key] = $item;
}

ksort($arr, SORT_NUMERIC);

However, it will drop inner arrays without an id.

E.g.

$old_arr = array(
    'a' => array ( 'id' => 20, 'name' => 'chimpanzee' ),
    'b' => array ( 'id' => 40, 'name' => 'meeting' ),
    'c' => array ( 'id' => 20, 'name' => 'dynasty' ),
    'd' => array ( 'id' => 50, 'name' => 'chocolate' ),
    'e' => array ( 'id' => 10, 'name' => 'bananas' ),
    'f' => array ( 'id' => 50, 'name' => 'fantasy' ),
    'g' => array ( 'id' => 50, 'name' => 'football' ),
    'h' => array ( 'name' => 'bob' )
);

will drop the 'h' array completely.