PhD, Startup, or Career? 
kin: You should ask yourself what you are really passionate about. I'm currently going through similar options and here's what I have to say.If you can do PhD later, then do it later. If it's an opportunity that you must act upon immediately, then do so. You'll double-triple your salary when you re-enter the corporate job market. However, keep in mind the being a graduate student is being an adult with no money. You'll have to tough it out for a few years.The same situation lies with your start-up. You've already let out your idea, therefore you must invest a large amount of time and effort (money) to release before any competition gets there first. At that point, what are the odds of your success?To be really cautious, I would personally go with the PhD and tough it out. If you're confident enough in your talent, you can develop for your start-up on the side. If, for some reason you fail at either or both, when you re-enter the job market, you'll still be worth gold.