我选择从父id= 0开始的所有category元素，并将其父id值等于父节点的id的项放入。

public function get_categories($table_name, $parent_id = 0)
{

$this->db->where('`parent`', $parent);

$parent = $this->db->get($table_name);

$categories = $parent->result();

$i = 0;
foreach ($categories as $p_cat) {

    $categories[$i]->sub = $this->sub_categories($table_name, $p_cat->id);
    $i++;
}
return $categories;
}

public function sub_categories($table_name, $id)
{

    $this->db->where('`parent`', $id);

    $child = $this->db->get($table_name);
    $categories = $child->result();
    $i = 0;
    foreach ($categories as $p_cat) {

        $categories[$i]->sub = $this->sub_categories($table_name, $p_cat->id);
        $i++;
    }
    return $categories;
}

这是my_categories数据表的输出。$categories内容如下：

Array ( 
    [0] => stdClass Object ( 
            [id] => 1 
            [name] => XMLELEMENT10 
            [parent] => 0 
            [sub] => Array ( 
                [0] => stdClass Object ( 
                    [id] => 13 
                    [name] => XMLELEMENT113 
                    [parent] => 1 
                    [sub] => Array ( ) 
                ) 
                [1] => stdClass Object ( 
                    [id] => 14 
                    [name] => XMLELEMENT114 
                    [parent] => 1 
                    [sub] => Array ( ) 
                ) 
                [2] => stdClass Object ( 
                    [id] => 15 
                    [name] => XMLELEMENT115
                    [parent] => 1 
                    [sub] => Array ( ) ) 
                [3] => stdClass Object ( 
                    [id] => 16 
                    [name] => XMLELEMENT116 
                    [parent] => 1 
                    [sub] => Array ( ) 
                ) 
            ) 
        ) 
    [1] => stdClass Object ( 
            [id] => 2 
            [name] => XMLELEMENT20 
            [parent] => 0 
            [sub] => Array ( 
                [0] => stdClass Object ( 
                    [id] => 5 
                    [name] => XMLELEMENT52 
                    [parent] => 2 
                    [sub] => Array ( ) 
                ) 
            ) 
        ) 
    [2] => stdClass Object ( 
            [id] => 3 
            [name] => XMLELEMENT30 
            [parent] => 0 
            [sub] => Array ( 
                [0] => stdClass Object ( 
                    [id] => 6 
                    [name] => XMLELEMENT63 
                    [parent] => 3 
                    [sub] => Array ( ) 
                ) 
                [1] => stdClass Object ( 
                    [id] => 7 
                    [name] => XMLELEMENT73 
                    [parent] => 3 
                    [sub] => Array ( ) 
                ) 
                [2] => stdClass Object ( 
                    [id] => 8 
                    [name] => XMLELEMENT83 
                    [parent] => 3 
                    [sub] => Array ( ) 
                ) 
                [3] => stdClass Object ( 
                    [id] => 9 
                    [name] => XMLELEMENT93 
                    [parent] => 3 
                    [sub] => Array ( ) 
                ) 
            ) 
        ) 
        [3] => stdClass Object ( 
            [id] => 4 
            [name] => XMLELEMENT40
            [parent] => 0 
            [sub] => Array ( 
                [0] => stdClass Object ( 
                    [id] => 10 
                    [name] => XMLELEMENT104 
                    [parent] => 4 
                    [sub] => Array ( ) 
                ) 
                [1] => stdClass Object ( 
                    [id] => 11 
                    [name] => XMLELEMENT2017114 
                    [parent] => 4 
                    [sub] => Array ( ) 
                ) 
                [2] => stdClass Object ( 
                    [id] => 12 
                    [name] => XMLELEMENT122 
                    [parent] => 4 
                    [sub] => Array ( ) 
                ) 
            ) 
        ) 
        [4] => stdClass Object ( 
            [id] => 17 
            [name] => XMLELEMENT170 
            [parent] => 0 
            [sub] => Array ( 
                [0] => stdClass Object ( 
                    [id] => 18 
                    [name] => XMLELEMENT1817 
                    [parent] => 17 
                    [sub] => Array ( ) 
                ) 
                [1] => stdClass Object ( 
                    [id] => 19 
                    [name] => XMLELEMENT1917 
                    [parent] => 17 
                    [sub] => Array ( ) 
                ) 
                [2] => stdClass Object ( 
                    [id] => 20 
                    [name] => XMLELEMENT2017 
                    [parent] => 17 
                    [sub] => Array ( ) 
                ) 
            ) 
        ) 
    )

问题很简单，但我不知道。我只需要列出所有项目，如

<ul>
    <li>root
     <!-- if has children -->
        <ul>
            <li>child
               <!-- if has children -->
                <ul>
                   <li>child of child
.
.
.

        </ul>
    </li>
</ul>

我无法编写简单递归函数。