MySQLi could be more handy in this, since you are using mysql as a database, but PDO is also helpful as an ORM tool, you can use it to abstract what kind of database you are using.

There is a tutorial for transactions in MySQL that could explain it better then I could.I haven't use transactions in MySQL, it should be the same as others but better informed then sorry.

I found this tutorial with a simple example for MySQLi, it might help.

I don't know much about the subject (I should learn mysqli soon though :S), but I hope this helps.