Online PHP editor | output for L4e6P

<?php $array = [ [ 'id' => 1, 'parentId' => "" ], [ 'id' => 2, 'parentId' => 1 ], [ 'id' => 3, 'parentId' => 2 ] ]; $entriesById = array_column($array, null, 'id'); $entriesWithAncestors = array_reduce( $entriesById, static function (array $result, array $entry) use ($entriesById): array { $result[$entry['id']] = $entry + ['ancestors' => []]; $parentId = $entry['parentId']; while (isset($result[$parentId])) { $result[$entry['id']]['ancestors'][] = $result[$parentId]; $parentId = $entriesById[$parentId]['parentId'] ?? null; } return $result; }, [] ); print_r($entriesWithAncestors);