Converting a flat array into a nested tree hierarchy

Given a linear array of elements representing tree nodes. Each element contains a unique identifier id and a reference to its parent node parent, which can be null (root node) or point to the id of another element. The task is to assemble a complete tree of arbitrary depth, placing child elements inside the children array of their parent.

/**
 * @param array{int, array{id: int, name: string, parent: ?int}} $flatList
 *
 * @return array{int, array{id: int, name: string, parent: ?int, children: array{int, array}}}
 */
function mapTree(array $flatList): array
{
    // implement conversion here
}

function testBuildTree(): void
{
    $source = [
        0 => ['id' => 0, 'parent' => null], // root
        1 => ['id' => 1, 'parent' => null], // root
        2 => ['id' => 2, 'parent' => 1],    // child of 1
        3 => ['id' => 3, 'parent' => 1],    // child of 1
        4 => ['id' => 4, 'parent' => 2],    // grandchild of 1
        5 => ['id' => 5, 'parent' => 4],    // great-grandchild of 1
    ];

    $resultTree = [
        0 => ['id' => 0, 'parent' => null],
        1 => [
            'id' => 1, 'parent' => null, 'children' => [
                0 => ['id' => 2, 'parent' => 1, 'children' => [
                    0 => ['id' => 4, 'parent' => 2, 'children' => [
                        0 => ['id' => 5, 'parent' => 4],
                    ]],
                ]],
                1 => ['id' => 3, 'parent' => 1],
            ],
        ],
    ];

    assert(mapTree($source) === $resultTree);
}

testBuildTree();