<?php
header("Content-type:text/html;charset=utf-8");
$city=array(
array('id'=>1,'name'=>'北京',"pid"=>0),
array('id'=>2,'name'=>'广东',"pid"=>0),
array('id'=>3,'name'=>'海淀区',"pid"=>1),
array('id'=>4,'name'=>'昌平区',"pid"=>1),
array('id'=>5,'name'=>'上地',"pid"=>3),
array('id'=>6,'name'=>'西二旗',"pid"=>3),
array('id'=>7,'name'=>'回龙观',"pid"=>4),
array('id'=>8,'name'=>'霍营',"pid"=>4),
array('id'=>9,'name'=>'深圳',"pid"=>2),
array('id'=>10,'name'=>'广州',"pid"=>2),
array('id'=>11,'name'=>'福田',"pid"=>9),
array('id'=>12,'name'=>'南山',"pid"=>9),
);
/*
$res=array(
array('id'=1,'name'=>'北京',"pid"=>0),
array('id'=3,'name'=>'海淀区',"pid"=>1),
array('id'=5,'name'=>'上地',"pid"=>3),
array('id'=6,'name'=>'西二旗',"pid"=>3),
array('id'=4,'name'=>'昌平区',"pid"=>1),
array('id'=7,'name'=>'回龙观',"pid"=>4),
array('id'=8,'name'=>'霍营',"pid"=>4),
array('id'=2,'name'=>'广东',"pid"=>0),
array('id'=9,'name'=>'深圳',"pid"=>2),
array('id'=11,'name'=>'福田',"pid"=>9),
array('id'=12,'name'=>'南山',"pid"=>9),
array('id'=10,'name'=>'广州',"pid"=>2),
);
*/
function tree($arr,$pid=0){
static $res=array();
foreach($arr as $v){
if($v['pid']==$pid){
//说明找到,先保存，继续递归查找
$res[]=$v;
tree($arr,$v['id']);
}
}
return $res;
}
echo "<pre>";
var_dump($city);
echo "<hr>";
$result=tree($city);
var_dump($result);

*********************************************************************************************************

在模型类中的使用情形：

/**
* @param array $arr [要排序的数组]
* @param integer $pid [父id]
* @return array [排序好的数组]
*/
public function tree($arr,$pid=0,$level=0){
       static $res=array();
       foreach($arr as $v){
       if($v['pid']==$pid){
          //说明找到，先保存，然后在递归查找

$v['level']=$level;

         $res[]=$v;
         $this->tree($arr,$v['cat_id'],$level+1);
      }
   }
  return $res;
}