配列リストを階層ツリー構造に効率的に変換するにはどうすればよいですか?

[
  {id: 100, parentId: 0, name: 'a'},
  {id: 101, parentId: 100, name: 'a'},
  {id: 102, parentId: 101, name: 'a'},
  {id: 103, parentId: 101, name: 'a'},
]

<?php

$arr = [
    ['id' => 100, 'parentId' => 0, 'name' => 'a'],
    ['id' => 101, 'parentId' => 100, 'name' => 'a'],
    ['id' => 102, 'parentId' => 101, 'name' => 'a'],
    ['id' => 103, 'parentId' => 101, 'name' => 'a'],
];

$new = [];
foreach ($arr as $a) {
    $new[$a['parentId']][] = $a;
}
$tree = createTree($new, [$arr[0]]);
print_r($tree);

function createTree(&$list, $parent) {
    $tree = [];
    foreach ($parent as $k => $l) {
        if (isset($list[$l['id']])) {
            $l['children'] = createTree($list, $list[$l['id']]);
        }
        $tree[] = $l;
    }
    return $tree;
}