关于mysql事务的问题

[tottyandbaty]

代码如下：

<?php
 
 $dsn  =  'mysql:dbname=test;host=192.168.0.72' ;
 $user  =  'root' ;
 $password  =  'root' ;

try {
     $dbh  = new  PDO ( $dsn ,  $user ,  $password );
} catch ( PDOException $e ) {
    echo  'Connection failed: '  .  $e -> getMessage ();
}
for($i=0;$i<=20;$i++){ 
	  printf('loop:%d<br>',$i);
	  trigger($dbh); 
}


function trigger($dbh){
	try{
		$dbh -> beginTransaction ();
		$rs=$dbh->exec("update account set money=money-10 where user_id=100");
		printf('exec:%d<br>',$rs);
		printf('start rollback:%d<br>',__LINE__); 
		$dbh->rollBack ();
                //这里为了测试，故意抛出异常
		throw new Exception('errror 24');  
		$dbh -> commit ();  
		
	} catch (Exception $e) { 
		printf('start rollback:%d<br>',__LINE__);
		$dbh->rollBack(); 
		echo  'Connection failed2: '  .  $e -> getMessage ();
	} 

}

?>

代码中有2个$dbh->rollBack(); 为啥循环只执行了1次？