Return an array with numeric keys PHP?

For this, use foreach loop along with [] and implement arary_values() method.

Example

<!DOCTYPE html>
<html>
<body>
<?php
$details = [
   [
      "id" => "10001",
      "firstName" => "John",
      "lastName" => "Doe"
   ],
   [
      "id" => "10002",
      "firstName" => "David",
      "lastName" => "Miller"
   ]
];
foreach ($details as $k=> $v) {
   $someDetails[$v['id']][] = ['LASTNAME' => $v['lastName'], 'FIRSTNAME'=> $v['firstName']];
}
print_r(array_values($someDetails));
?>
</body>
</html>

Output

Array ( [0] => Array ( [0] => Array ( [LASTNAME] => Doe [FIRSTNAME] => John ) ) [1] => Array ( [0] => Array ( [LASTNAME] => Miller [FIRSTNAME] => David ) ) )