PHP array tree task

output.json

{
    "name": "A",
    "parent": null,
    "id": 1,
    "children": [
        {
            "name": "B",
            "parent": "1",
            "id": 2,
            "children": [
                {
                    "name": "B1",
                    "parent": "2",
                    "id": 3,
                    "children": [
                        {
                            "name": "B2",
                            "parent": "3",
                            "id": 4,
                            "children": [
                                {
                                    "name": "B3",
                                    "parent": "4",
                                    "id": 5,
                                    "children": []
                                }
                            ]
                        }
                    ]
                }
            ]
        },
        {
            "name": "C",
            "parent": "1",
            "id": 6,
            "children": [
                {
                    "name": "C1",
                    "parent": "6",
                    "id": 7,
                    "children": [
                        {
                            "name": "C2",
                            "parent": "7",
                            "id": 8,
                            "children": [
                                {
                                    "name": "C3",
                                    "parent": "8",
                                    "id": 9,
                                    "children": []
                                }
                            ]
                        }
                    ]
                }
            ]
        }
    ]
}

task.php

<?php

// Example single fork
$data = [

    //Top of Tree
    [
        "name" => "A",
        "parent" => null,
        "id" => 1,
    ],

    //B Branch
    [
        "name" => "B",
        "parent" => "1",
        "id" => 2,
    ],
    [
        "name" => "B1",
        "parent" => "2",
        "id" => 3,
    ],
    [
        "name" => "B2",
        "parent" => "3",
        "id" => 4,
    ],
    [
        "name" => "B3",
        "parent" => "4",
        "id" => 5,
    ],

    //C Branch
    [
        "name" => "C",
        "parent" => "1",
        "id" => 6,
    ],
    [
        "name" => "C1",
        "parent" => "6",
        "id" => 7,
    ],
    [
        "name" => "C2",
        "parent" => "7",
        "id" => 8,
    ],
    [
        "name" => "C3",
        "parent" => "8",
        "id" => 9,
    ],

];


function buildTree(array $items): ?array {

    // Get a mapping of each item by ID, and pre-prepare the "children" property.
    $idMap = [];
    foreach ($items as $item) {
        $idMap[$item['id']] = $item;
        $idMap[$item['id']]['children'] = [];
    }

    // Store a reference to the treetop if we come across it.
    $treeTop = null;

    // Map items to their parent's children array.
    foreach ($idMap as $id => $item) {
        if ($item['parent'] && isset($idMap[intval($item['parent'])])) {
            $parent = &$idMap[intval($item['parent'])];
            $parent['children'][] = &$idMap[$id];
        } else if ($item['parent'] === null) {
            $treeTop = &$idMap[$id];
        }
    }

    return $treeTop;
}

//echo json_encode($data, JSON_PRETTY_PRINT);
echo json_encode(buildTree($data), JSON_PRETTY_PRINT);