PDO事务返回错误未捕获的PDOException：：55中没有活动事务

Question

我的连接文件如下所示

static public function connect(){

    $link = new PDO("mysql:host=localhost;dbname=xx", "xx", "xx");
            $link->setAttribute(pdo::ATTR_AUTOCOMMIT, 0);
    $link -> exec("set names utf8");

    return $link;
}

} }

“我的带有嵌套数据库的代码插入和更新”是以下代码

$inv_no=1047;

            $chick_id = 1;
            $chick_price = 15;
            $chick_qty = 1000;
            $sup_id=3;

            $sub_total=$chick_price*$chick_qty;

        $current= new ProcsModel();
        $inv_info=$current->Mdl_Invtotal_Itemcount($inv_no);
        $sup_info=$current->Mdl_Sup_Total($sup_id);

        $sup_cur_balance=$sup_info['balance'];
        $sup_new_balance=$sup_cur_balance+$sub_total;

        $current_inv_total=$inv_info['inv_total'];
        $current_item_count=$inv_info['item_count'];

        $new_count=$current_item_count+1;
        $new_inv_total=$current_inv_total+$sub_total;



        $stmt = Connection::connect()->beginTransaction();

        // Add Item
        $stmt = Connection::connect()->prepare("INSERT INTO `pro_items` (inv_no, chick_id, chick_price, chick_qty, sub_total) VALUES ($inv_no, $chick_id, $chick_price, $chick_qty, $sub_total )");


    if ($stmt->execute()) {
        //Update Invoice Table
                    $stmt = Connection::connect()->prepare("UPDATE pro_inv set inv_total = $new_inv_total, item_count = $new_count WHERE inv_no = $inv_no");

                            if ($stmt->execute()) {
                            //Update Suppliers Table
                            $stmt = Connection::connect()->prepare("UPDATE suppliers set balance = $sup_new_balance WHERE sup_id = $sup_id");

                                                                if ($stmt->execute()) {

                                                                 $stmt = Connection::connect()->commit();                   

                                                                 $chick_type=ProcsModel::Mdl_Get_chick_type($chick_id);

                                                                $item= array ('item_count'=>$new_count,
                                                                            'chick_type'=>$chick_type,
                                                                            'chick_price'=>$chick_price,
                                                                            'sub_total'=>$$sub_total,
                                                                            'inv_total'=>$new_inv_total,
                                                                            'chick_qty'=>$chick_qty);

                                                                var_dump ($item);    

                                                                                        }
                                                                else{
                                                               $stmt = Connection::connect()->rollBack();
                                                               return 'error';
                                                               exit();
                                                                   }
                    }
                            else{
                                $stmt = Connection::connect()->rollBack();
                                return 'error';
                                exit();
                                }
    } 
            else {

        $stmt = Connection::connect()->rollBack();
                    return 'error';
                    exit();

    }

    $stmt -> close();

    $stmt = null;

运行此命令时，出现错误Uncaught PDOException：我提交事务第55行的行中没有活动事务

任何人都可以帮助我弄清楚为什么我会收到此错误