I've worked in over twenty languages in my career, and you can't pay me to work with PHP anymore, even though there was a brief period where I thought it was the best thing ever as a new programmer. But that's just an artist being picky about his tools.
What matters with a startup is executing, and that means using tools that let you execute well. If you're most familiar with PHP and MySQL, then that's what you should use.
As someone biased against your tech stack, I fully support your decision and think the expert and the investor have no idea what really matters, hence I stand by that they're a bad investor.
