Question

EDITED：从现在开始我知道这被称为递归函数，我从这里找到了更多信息：

What is a RECURSIVE Function in PHP?

我可以让var_dump为数组工作，但无法使返回函数起作用。返回值始终为NULL。有人可以帮忙吗？

预期结果：

我从var_dump得到的

是：

array（3）{[0] =＆gt; string（23）“ - range”[1] =＆gt; string（22）“ - fruit”[2] =＆gt; string（22）“ - plant”}

CREATE TABLE IF NOT EXISTS `level_test` (
`id` int(11) NOT NULL,
`name` varchar(80) NOT NULL,
`parentid` int(11) NOT NULL
) ENGINE=MyISAM AUTO_INCREMENT=8 DEFAULT CHARSET=utf8;

INSERT INTO `level_test` (`id`, `name`, `parentid`) VALUES
(1, 'plant', 0),
(2, 'fruit', 1),
(3, 'vegetable', 1),
(4, 'apple', 2),
(5, 'orange', 2),
(6, 'tomatoe', 3),
(7, 'cabbage', 3);

ALTER TABLE `level_test`
ADD PRIMARY KEY (`id`);

ALTER TABLE `level_test`
MODIFY `id` int(11) NOT NULL AUTO_INCREMENT,AUTO_INCREMENT=8;
/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
/*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */;
/*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */;


function find_tree($pdcatid, $tree ="") {
    global $con;
    if ($tree == '') {
        $tree = array();
    }
    $query_level = "SELECT * from level_test WHERE `id` = $pdcatid";
    if ($result = $con->query($query_level)) {
        while($row = $result->fetch_array()) {
            if ($result->num_rows == 1) {
                array_push($tree, '-<a href="'.$row['id'].'">'.$row['name'].'</a>');
                if ($row['parentid'] == 0) {
                    var_dump($tree); // this works
                    return $tree; // this doesn't
                    echo 'hello'; // just for troubleshoot which it doesn't echo
                } else {
                    $parentid = $row['parentid'];
                    find_tree($parentid, $tree);
                }
            }
        }
    }
}

$result_tree = find_tree(5);
var_dump($result_tree);

Answer 1

返回递归find_tree结果，如下所示：

if ($row['parentid'] == 0) {
    var_dump($tree); // this works
    return $tree; // this doesn't
    echo 'hello'; // just for troubleshoot which it doesn't echo
} else {
    $parentid = $row['parentid'];
    return find_tree($parentid, $tree); // <-- HERE
}

现在，仅当$row['parentID']为0时才返回值。如果parentID不为0，则不返回任何值（NULL）。所有void函数都返回NULL。然后，如果你调用non-void函数递归，你必须在父调用中返回它的结果。

RECURSIVE函数返回数组为NULL，而返回前的var_dump工作

1 个答案: