If an PHP PDO transaction fails, must I rollback() explicitely?

I've seen an code example whe开发者_如何学编程re someone does a

$dbh->rollback();

when there occurs an PDOException. I thought the database will rollback automatically in such a case?

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If you don't commit not rollback an opened transaction, and it's not commited anywhere later in your script, it won't be commited (as seen by the database engine), and will automatically rolled-back at the end of your script.

Still, I (well, almost) always commit or rollback explicitly the transactions I open, so :

• There is not risk of an error (like commiting "by mistake" later in the script)
• The code is more easy to read / understand : when one sees $db->rollback(), he knows I want the transaction rolled-back for sure, and he doesn't have to think "did he really want to rollback, or did he forget something ? and what about later in the script ?"

The DB engine doesn't "see" the PDOException : it is thrown by PHP under various conditions -- but the database doesn't rollback anything by itself :

• either a transaction is commited
• or it's rolled-back
• or it's not explicitly commited nor rolled-back -- which means it's not commited -- which means what's been modified is not "really" modified