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<?php declare(strict_types=1); $items = [ (object)['id' => 0, 'name' => "root"] (object)['id' => 1, 'parentId' => 0, 'name' => "foo"], (object)['id' => 2, 'parentId' => 1, 'name' => "bar"], (object)['id' => 3, 'parentId' => 1, 'name' => "qux"], (object)['id' => 4, 'parentId' => 3, 'name' => "zip"], (object)['id' => 5, 'parentId' => 4, 'name' => "baz"] ]; $itemsById = []; foreach ($items as $item) { $itemsById[$item->id] = $item; } // --- foreach ($itemsById as $id => $item) { if (isset($item->parentId)) { $itemsById[$item->parentId]->children[] = $item; } } $root = $itemsById[0]; var_dump($root);