为什么这个递归函数只渲染第一级深度？

Question

 //here i am getting the parent_id of this root node
 foreach($query->result() as $q){


        if($q->first == 'first')
        {
            $parent_id = $q->parent_id;
            break;
        }


    }
function print_list($array, $parent=0) {
        print "<ul>";
        foreach ($array as $row) {
            if ($row->parent_id == $parent) {
                print "<li>$row->FIO";
                print_list($array, $row->id);  # recurse
                print "</li>";
        }   }
        print "</ul>";
    }
echo print_list($query->result(), $parent_id);

为什么它只呈现第一级深度？

How to build a tree in php having id, parent_id and depth variables - 此处的功能

的var_dump：

     //var_dump($query->result());
array(4) {
  [0]=>
  object(stdClass)#18 (5) {
    ["id"]=>
    string(3) "501"
    ["parent_id"]=>
    string(1) "1"
    ["FIO"]=>
    string(23) "Пётр Василий"
    ["depth"]=>
    string(1) "0"
    ["first"]=>
    string(5) "first"
  }
  [1]=>
  object(stdClass)#19 (5) {
    ["id"]=>
    string(3) "503"
    ["parent_id"]=>
    string(3) "501"
    ["FIO"]=>
    string(33) "Джумшут Джумшутов"
    ["depth"]=>
    string(1) "1"
    ["first"]=>
    NULL
  }
  [2]=>
  object(stdClass)#20 (5) {
    ["id"]=>
    string(3) "504"
    ["parent_id"]=>
    string(3) "501"
    ["FIO"]=>
    string(7) "dwadawd"
    ["depth"]=>
    string(1) "1"
    ["first"]=>
    NULL
  }
  [3]=>
  object(stdClass)#21 (5) {
    ["id"]=>
    string(3) "505"
    ["parent_id"]=>
    string(3) "501"
    ["FIO"]=>
    string(6) "vasekd"
    ["depth"]=>
    string(1) "1"
    ["first"]=>
    NULL
  }
}

这是显示var_dump（$ query-＆gt; result（））;

的内容