In theory, I think you need to be VERY sure you want to start a PhD, because once you start it, you should to be motivated enough to complete it successfully. If you're not motivated enough, it will probably be a nightmare.
But
if you're very ambitious and...a bit masochist, I'd say: "do it, start the PhD, finish it and get on with your life" even if you're not sure or you think you're not sure.