Flat To Tree

flatToTree.php

<?php

$flat = [
    ['id' => 1, 'parentId' => 0],
    ['id' => 2, 'parentId' => 0],
    ['id' => 3, 'parentId' => 1],
    ['id' => 4, 'parentId' => 1],
    ['id' => 5, 'parentId' => 2],
    ['id' => 6, 'parentId' => 2],
    ['id' => 7, 'parentId' => 3],
    ['id' => 8, 'parentId' => 0],
];

$expected = [
    1 => ['id' => 1, 'parentId' => 0, 'child' => [
        ['id' => 3, 'parentId' => 1, 'child' => [
            ['id' => 7, 'parentId' => 3],
        ]],
        ['id' => 4, 'parentId' => 1],
    ]],
    2 => ['id' => 2, 'parentId' => 0, 'child' => [
        ['id' => 5, 'parentId' => 2],
        ['id' => 6, 'parentId' => 2],
    ]],
    8 => ['id' => 8, 'parentId' => 0],
];

class FlatToTree
{

    public function convertArray(array $data): array
    {
        $map = [];
        foreach ($data as $item) {
            $map[$item['id']] = $item;
        }

        foreach ($map as &$item) {
            $parentId = $item['parentId'] ?? 0;
            if ($parentId !== 0) {
                $map[$parentId]['child'][] = &$item;
            }
        }

        return array_filter($map, fn(array $item): bool => $item['parentId'] === 0);
    }
}

$service = new FlatToTree();
$result = $service->convertArray($flat);

echo print_r($result, true) === print_r($expected, true) ? 'ok' : 'not ok';