function tran(){
  //定义事务成功失败的标志
  $mark = true;
  //1. 实例化模型
  $model = D('student');
  //2. 开启事务处理
  $model->startTrans();
  //3. ls减少2000
  $sql = "update student set money=money-2000 where uname='ls'";
  $result = $model->execute($sql);
  //判断sql执行是否成功,如果失败，则将$mark改为false
  if(!$result){
    $mark = false;
  }
  //4. zs增加2000
  $sql = "update student set money=money+2000 where uname='zs'";
  $result = $user->execute($sql);
  //判断sql执行是否成功,如果失败，则将$mark改为false
  if(!$result){
    $mark = false;
  }
  $mark = false; //暂时强制改为false，测试回滚效果
  //5. 提交事务
  //判断$mark的值,为ture则提交，为false则回滚
  if($mark){
    $user->commit();
  } else {
    $user->rollback();
  }
}