PHP如何得到存储过程的返回值

$db=new mysqli("localhost","ssss","aaaaa","bbbb");
mysqli_query($db,"SET NAMES utf8");
$result=$db->query("call gxtj($year,$jd)"); // gxtj是mysql的存储过程名称
while( $row = $result->fetch_array(MYSQLI_ASSOC)) //完成从返回结果集中取出一行

{
while ($key=key($row)){ //依次取得字段名
$value=current($row); //依次取得字段值
}
}