Most of what I say here probably applies more to sci than commerce, but I think it's generally valid across all the faculties.
Now, you should really only be consider a PhD if you absolutely love and are dedicated to your area of research. Most people don't even consider them until at least they've had a taste for research - usually in their honours year. A PhD is effectively a lonely 3-4 year hard slog where everything you do and breathe will be your research project - which will most likely be in some super specialized field. It's not like some kind of job which you can just leave at any time either if you don't like the conditions. No, if you find your research area boring, then there's no way you can just dump your project and take up another one - you're stuck till you finish it.
It's good to have goals, but if you are seriously into business then you should really be thinking about what area of it you would like to major in rather than a PhD atm. Like I said above, if you are still really keen then at least get a taste for what it will be like by taking an honours year.
In any case, people often change their decisions on their career path many times through both during HS and at uni. In the end, you might find that even as fascinating as business it, maybe you would prefer to just get into the workforce and apply your knowledge (rather than do research).