PHP recursively traverses multi-dimensional arrays to achieve infinite classification_php tips

<&#63;php
   //$data[]=array('id'=>1,'parentid'=>0,'name'=>'中国','img'=>'52091199');
   $data[]=array('id'=>1,'parentid'=>0,'name'=>'中国');
   $data[]=array('id'=>2,'parentid'=>0,'name'=>'美国');
   $data[]=array('id'=>3,'parentid'=>0,'name'=>'韩国');
   $data[]=array('id'=>4,'parentid'=>1,'name'=>'北京');
   $data[]=array('id'=>5,'parentid'=>1,'name'=>'上海');
   $data[]=array('id'=>6,'parentid'=>1,'name'=>'广西');
   $data[]=array('id'=>7,'parentid'=>6,'name'=>'桂林');
   $data[]=array('id'=>8,'parentid'=>6,'name'=>'南宁');
   $data[]=array('id'=>9,'parentid'=>6,'name'=>'柳州');
   $data[]=array('id'=>10,'parentid'=>2,'name'=>'纽约');
   $data[]=array('id'=>11,'parentid'=>2,'name'=>'华盛顿');
   $data[]=array('id'=>12,'parentid'=>3,'name'=>'首尔');
   $tree=build_tree($data,0);
   //echo memory_get_usage();
   print_r($tree);
 function findChild(&$arr,$id){
  $childs=array();
  foreach ($arr as $k => $v){
   if($v['parentid']== $id){
    $childs[]=$v;
   }
  }
  return $childs;
 }
 function build_tree($rows,$root_id){
  $childs=findChild($rows,$root_id);
  if(empty($childs)){
   return null;
  }
 foreach ($childs as $k => $v){
  $rescurTree=build_tree($rows,$v['id']);
  if( null != $rescurTree){
  $childs[$k]['childs']=$rescurTree;
  }
 }
  return $childs;
 }
&#63;>

Array
(
 [0] => Array
  (
   [id] => 1
   [parentid] => 0
   [name] => 中国
   [childs] => Array
    (
     [0] => Array
      (
       [id] => 4
       [parentid] => 1
       [name] => 北京
      )
     [1] => Array
      (
       [id] => 5
       [parentid] => 1
       [name] => 上海
      )
     [2] => Array
      (
       [id] => 6
       [parentid] => 1
       [name] => 广西
       [childs] => Array
        (
         [0] => Array
          (
           [id] => 7
           [parentid] => 6
           [name] => 桂林
          )
         [1] => Array
          (
           [id] => 8
           [parentid] => 6
           [name] => 南宁
          )
         [2] => Array
          (
           [id] => 9
           [parentid] => 6
           [name] => 柳州
          )
        )
      )
    )
  )
 [1] => Array
  (
   [id] => 2
   [parentid] => 0
   [name] => 美国
   [childs] => Array
    (
     [0] => Array
      (
       [id] => 10
       [parentid] => 2
       [name] => 纽约
      )
     [1] => Array
      (
       [id] => 11
       [parentid] => 2
       [name] => 华盛顿
      )
    )
  )
 [2] => Array
  (
   [id] => 3
   [parentid] => 0
   [name] => 韩国
   [childs] => Array
    (
     [0] => Array
      (
       [id] => 12
       [parentid] => 3
       [name] => 首尔
      )
    )
  )
)