//二维数组转字符串 
public function arr_to_str($arr){ 
   foreach ($arr as $v){ 
       $v = join(",",$v); //可以用implode将一维数组转换为用逗号连接的字符串，join是别名 
       $temp[] = $v; 
   } 
   foreach($temp as $v){ 
      $t.=$v.","; 
   } 
   $t=substr($t,0,-1); //利用字符串截取函数消除最后一个逗号 
   return $t; 
}