If I end up with too much beer and not enough bottles and I'm really desperate, I usually dump one of my wife's bottles of Club Soda / Diet Coke from the fridge, and buy it back later. The PET bottles are definitely built for carbonation, since it's original contents are far more carbed than your beer. If it contained something like Coke, it's practically disinfected already, since nothing survives in that acidity.