PHP树形数组遍历：如何找出所有根节点及其子孙节点？

$tree = [
    ['node' => 2, 'children' => [3, 9, 4]],
    ['node' => 7, 'children' => [2]],
    ['node' => 3, 'children' => [6]],
    ['node' => 4, 'children' => [5]],
    ['node' => 5, 'children' => [8]],
    ['node' => 10, 'children' => [11]]
];

<?php
$tree = [
    ['node' => 2, 'children' => [3, 9, 4]],
    ['node' => 7, 'children' => [2]],
    ['node' => 3, 'children' => [6]],
    ['node' => 4, 'children' => [5]],
    ['node' => 5, 'children' => [8]],
    ['node' => 10, 'children' => [11]]
];

// 先将子元素合并
$children = [];
foreach ($tree as $k => $v) {
    $children = array_merge($children, $v['children']);
}

// 找出第一层
$loop = [];
foreach ($tree as $item) {
    if (!in_array($item['node'], $children)) {
        $loop[] = $item['node'];
    }
}

// 递归找结果
function recursion(&$tree, $loop)
{
    static $result = [];
    $result = array_merge($result, $loop);

    if ($tree) {
        $next_children = [];
        foreach ($tree as $k => $item) {
            if (in_array($item['node'], $loop)) {
                $next_children = array_merge($next_children, $item['children']);
                unset($tree[$k]);
            }
        }
        return recursion($tree, $next_children);
    } else {
        return $result;
    }
}

$result = recursion($tree, $loop);

print_r($result);

// 结果
// Array ( [0] => 7 [1] => 10 [2] => 2 [3] => 11 [4] => 3 [5] => 9 [6] => 4 [7] => 6 [8] => 5 [9] => 8 )
?>