Yeah, absolutely, don't do this! These are two examples of successful companies that did it and look at them now! </s>
Being able to move fast and produce a winning product on time is much more important for startups. What does it matter if you used <your_cool_scalable_thingie> for a project, when it never went past 10 users because you were concentrating on wrong side aspect of your business? PHP is fine. Python is great. Use the tools that fit your problem and you know how to use, not the latest toy.