Group rows in an associative array of associative arrays by column value and preserve the original first level keys

Question

I have an array of subarrays in the following format:

[
    'a' => ['id' => 20, 'name' => 'chimpanzee'],
    'b' => ['id' => 40, 'name' => 'meeting'],
    'c' => ['id' => 20, 'name' => 'dynasty'],
    'd' => ['id' => 50, 'name' => 'chocolate'],
    'e' => ['id' => 10, 'name' => 'bananas'],
    'f' => ['id' => 50, 'name' => 'fantasy'],
    'g' => ['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 )
          )
)

Answer 1

$arr = array();

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

ksort($arr, SORT_NUMERIC);