php简单实现无限分类树形列表的方法

$items = array( 
  1 => array('id' => 1,'pid' => 0,'name' => '江西省'),2 => array('id' => 2,'name' => '黑龙江省'),3 => array('id' => 3,'pid' => 1,'name' => '南昌市'),4 => array('id' => 4,'pid' => 2,'name' => '哈尔滨市'),5 => array('id' => 5,'name' => '鸡西市'),6 => array('id' => 6,'pid' => 4,'name' => '香坊区'),7 => array('id' => 7,'name' => '南岗区'),8 => array('id' => 8,'pid' => 6,'name' => '和兴路'),9 => array('id' => 9,'pid' => 7,'name' => '西大直街'),10 => array('id' => 10,'pid' => 8,'name' => '东北林业大学'),11 => array('id' => 11,'pid' => 9,'name' => '哈尔滨工业大学'),12 => array('id' => 12,'name' => '哈尔滨师范大学'),13 => array('id' => 13,'name' => '赣州市'),14 => array('id' => 14,'pid' => 13,'name' => '赣县'),15 => array('id' => 15,'name' => '于都县'),16 => array('id' => 16,'pid' => 14,'name' => '茅店镇'),17 => array('id' => 17,'name' => '大田乡'),18 => array('id' => 18,'pid' => 16,'name' => '义源村'),19 => array('id' => 19,'name' => '上坝村'),20 => array('id' => 20,'name' => '广东省'),21 => array('id' => 21,'pid' => 20,'name' => '广州市'),); 
$t = array(); 
foreach ($items as $id => $item) { 
  if ($item['pid']) { 
    $items[$item['pid']][$item['id']] = &$items[$item['id']];
    $t[] = $id; 
  } 
} 
foreach($t as $u) { 
  unset($items[$u]); 
} 
echo "<pre>"; 
print_r($items);