PHP_PDO call mysql stored procedures with return parameters

DROP PROCEDURE IF EXISTS pro_test;

create procedure pro_test(in val VARCHAR(50) ,out rtn int)
begin
declare err INT default 0;
-- 如果出现异常，会自动处理并rollback
declare exit handler for  sqlexception ROLLBACK ;

-- 启动事务
start transaction;

insert into test_user values(NULL,1,'啊是大三的');
-- set err = @@IDENTITY; -- =	获取上一次插入的自增ID;
set err =last_insert_id(); -- 获取上一次插入的自增ID
insert into test_user VALUES(NULL,val,err);

-- 运行没有异常，提交事务
commit;
-- 设置返回值为1
set rtn=1;
end;

$name = '成都市';
		
		$stmt = $db->prepare("CALL pro_test(?,@sp_result);");
		$stmt->bindParam(1, $name);
		$stmt->execute ();
		$outputArray = $db->query("select @sp_result")->fetch(PDO::FETCH_ASSOC);

				print_r($outputArray["@sp_result"]);