我创建了一个班级并采纳了你的想法。

class TreeBuilder {
    private $flatArr;
    private $idToNode;

    public function __construct($flatArr) {
        // Keep the flat arr in case we need it.
        $this->flatArr = $flatArr;
        // Create an array to lookup a node to determine if it exists.
        $this->idToNode = array_combine(array_column($flatArr, 'id'), $flatArr);
    }

    public function buildTree() {
        // create an empty array to hold root nodes
        $roots = [];
        // iterate through each node and add it to its parent's children list
        foreach ($this->flatArr as &$node) {
            $id = $node['id'];
            $parentId = $node['parentId'];
            if (isset($this->idToNode[$parentId])) {
                $this->out("add child to $parentId " . print_r($node, true));
                $parentNode = &$this->idToNode[$parentId];
                if ( isset($parentNode['children']) ) {
                    $parentNode['children'] = [&$this->idToNode[$id]];
                } else {
                    $parentNode['children'][] = &$this->idToNode[$id];
                }
                // $children[] = &$node;
            } else {
                $this->out("add to root " . print_r($node, true));
                $roots[] = &$this->idToNode[$id];
            }
        }
        // calculate the sum of each node and its children recursively
        foreach ($roots as &$root) {
            $this->calculateSum($root);
        }
        return $roots;
    }

    private function calculateSum(&$node) {
        // calculate the sum of the current node
        $node['sum'] = $node['cost'];
        // recursively calculate the sum of the children nodes
        $children = &$node['children'];
        if (isset($children)) {
            foreach ($children as &$child) {
                $node['sum'] += $this->calculateSum($child);
            }
        }
        return $node['sum'];
    }

    private function out($s) {
        echo "$s\n";
    }
}

你可以不使用递归来构建树，然后使用递归来更新和，按照后序深度的第一顺序：

function buildTree(array $flat) {
    foreach ($flat as ["id" => $id, "cost" => $sum]) {
        $keyed[$id] = ["id" => $id, "sum" => $sum];
    }
    foreach ($flat as ["id" => $id, "parentId" => $parentId]) {
        if (isset($keyed[$parentId])) {
            $keyed[$parentId]["children"][] = &$keyed[$id]; 
        } else {
            $root = &$keyed[$id];
        }
    }
    function updateSum(&$node) {
        foreach ($node["children"] ?? [] as &$child) {
            $node["sum"] += updateSum($child);
        }
        return $node["sum"];
    }
    updateSum($root);
    return $root;
}

运行示例：

$flat = json_decode('[
    {
        "id": "1",
        "parentId": "0",
        "cost": 1000
    },
    {
        "id": "2",
        "parentId": "1",
        "cost": 2000
    },
    {
        "id": "3",
        "parentId": "2",
        "cost": 4000
    }
]', true);

$root = buildTree($flat);
print_r($root);