How to execute two PDO statements in one function and also prioritize which to execute first?

Question

I'm attempting to move away from PHP mysql functions to something newer, either mysqli or PDO.

I have read various things about them, but I'm curious at this point which would be best to learn for the present and future. I'm usually working on small to medium size projects.

I understand that PDO has an extra abstraction layer so it can work with / convert to other databases like oracle and others fairly easily, but some have said that PDO can be a bit slower the MySQLi?

So I would really like to hear opinions on the best choices of either MySQLi or PDO.

Also I'm also trying to learn OOP, so I'm looking for a very good working PDO and/or mysqli database class - to get started with, that has all the basic needs like UPDATE - INSERT - DELETE - QUERY etc. That would be very helpful.