说明可以作为内联找到注释。此功能提供您确切的所需输出。还请注意我为设置 $ resultset 而重写的查询。

Explanations can be found as inline comments. This function provides your exact desired output. Also pay attention to the query that I've rewritten to set up $resultset.

代码：（演示）

function findParent(&$array,$parentid=0,$childarray=[]){  // make $array modifiable
    foreach($array as $i=>&$row){                         // make $row modifiable
        if($parentid){                                    // if not zero
            if($row['id']==$parentid){                    // found parent
                $row['nodes'][]=$childarray;              // append child to parent's nodes subarray
            }elseif(isset($row['nodes'])){                // go down rabbit hole looking for parent
                findParent($row['nodes'],$parentid,$childarray);  // look deeper for parent while preserving the initial parent_id and row
            }                                             // else continue;
        }elseif($row['parent_id']){                       // child requires adoption
            unset($array[$i]);                            // remove child from level because it will be store elsewhere and won't be its own parent (reduce iterations in next loop & avoid infinite recursion)
            findParent($array,$row['parent_id'],$row);    // look for parent using parent_id while carrying the entire row as the childarray
        }                                                 // else continue;
    }
    return $array;                                        // return the modified array
}


// $db->query('SELECT id,name_a AS name,parent_id FROM accounts_tree ORDER BY id');
// for($resultset=[]; $row=$res->fetch_assoc(); $resultset[]=$row);  // inspired by: http://php.net/manual/en/mysqli-result.fetch-assoc.php#112924

$resultset=[
    ['id'=>1,'name'=>'folder 1','parent_id'=>0],
    ['id'=>2,'name'=>'folder 2','parent_id'=>0],
    ['id'=>3,'name'=>'sub 1-1','parent_id'=>1],
    ['id'=>4,'name'=>'sub 2-1','parent_id'=>2],
    ['id'=>5,'name'=>'Sub 1-1-1','parent_id'=>3],
    ['id'=>6,'name'=>'folder 3','parent_id'=>0],
    ['id'=>7,'name'=>'sub 1-1-1-1','parent_id'=>5]
];

print_r(findParent($resultset));

输出：

Array
(
    [0] => Array
        (
            [id] => 1
            [name] => folder 1
            [parent_id] => 0
            [nodes] => Array
                (
                    [0] => Array
                        (
                            [id] => 3
                            [name] => sub 1-1
                            [parent_id] => 1
                            [nodes] => Array
                                (
                                    [0] => Array
                                        (
                                            [id] => 5
                                            [name] => Sub 1-1-1
                                            [parent_id] => 3
                                            [nodes] => Array
                                                (
                                                    [0] => Array
                                                        (
                                                            [id] => 7
                                                            [name] => sub 1-1-1-1
                                                            [parent_id] => 5
                                                        )

                                                )

                                        )

                                )

                        )

                )

        )

    [1] => Array
        (
            [id] => 2
            [name] => folder 2
            [parent_id] => 0
            [nodes] => Array
                (
                    [0] => Array
                        (
                            [id] => 4
                            [name] => sub 2-1
                            [parent_id] => 2
                        )

                )

        )

    [5] => Array
        (
            [id] => 6
            [name] => folder 3
            [parent_id] => 0
        )

)