Wann verwendet PHP Transaktionen?

<?php
	$link = mysqli_connect(&#39;localhost&#39;,&#39;username&#39;,&#39;password&#39;);					//创建链接

	if(!$link) exit(&#39;数据库连接失败&#39;);											//判断是否链接成功

	mysqli_set_charset($link , &#39;utf8&#39;);											//设置字符集
	
	mysqli_select_db($link,&#39;myDatabase&#39;);										//选择数据库

	$sql1 = "正确的插入语句";													//准备sql语句
	$sql2 = "错误的插入语句";													

	$result1 = mysqli_query($link , $sql1);										//发送sql语句
	$result2 = mysqli_query($link , $sql2);	

	if($result1 && $result2) {
		mysql_query(“COMMIT”);													//提交事务
		echo "提交成功";
	} else {
		mysql_query("ROLLBACK");
		echo &#39;数据回滚&#39;;
	}

	mysql_query("END");															//事务结束

try {

	 //实例化PDO对象
	 $pdo = new PDO("mysql:host=localhost;dbname=test","root","root",array(PDO::ATTR_ERRMODE=>PDO::ERRMODE_EXCEPTION));
   	 $pdo->setAttribute(PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
	
	 //开启事务
	 $pdo->beginTransaction();
	  
	 //执行sql语句
	 $pdo->exec("insert into t1(username,password,rtime,rip) values('shiwu2','shiwu2','456456456','456456456')");
     $pdo->exec("insert into t2(username,password,rtime,rip) values('shiwu2',shiwu2','456456456','456456456')");
     
     //提交事务
     $pdo->commit();
     //PDO   PDOStatement  PDOException
} catch(PDOException $e) {
	 //回滚事务
     $pdo->rollBack();
	 echo "数据回滚";    
}

//模型方法
function demo{
	//开启事务
	$this->startTrans();
	
	//业务逻辑操作
	$data['id'] = 1;
	$res = $this->insertUserInfo($data);		//保存用户信息
	
	if($res) {
		//提交事务
		$this->commit();
		return $res;
	} else {
		//事务回滚
		$this->rollback();
	}
}