Is it safe to give babies liquids in plastic bottles?

Yes. Yes, it is totally safe to give babies liquids in plastic bottles these days. Almost all plastic bottles are bpa free these days, so you shouldn't be worried about bpa. That being said, bpa in humans has yet to be found to cause any proven problems.
No. Many countries have banned the use of bpa´s in infant bottles, sippy cups and nipples for a reason. We have not. Try to use bottles and nipples that are bpa free or better yet try glass bottles if you can. Granted plastic is convenient and has saved us all in a pinch.