Online PHP editor | output for ofP5r

<?php function buildTree(array $rows, int $parentId = 0): array { $new = []; foreach ($rows as $row) { if ($row['parent_id'] == $parentId) { $new[$row['id']] = buildTree($rows, $row['id']) ?: ''; } } return $new; } $resultSet = [ ['id' => 1, 'parent_id' => 0], ['id' => 2, 'parent_id' => 1], ['id' => 3, 'parent_id' => 2], ['id' => 4, 'parent_id' => 2], ['id' => 5, 'parent_id' => 3], ['id' => 6, 'parent_id' => 3], ['id' => 7, 'parent_id' => 4], ['id' => 8, 'parent_id' => 4], ['id' => 9, 'parent_id' => 4], ]; var_export( buildTree($resultSet) );