Here's a broader look at tech stacks used by startups on Angel List: http://codingvc.com/which-technologies-do-startups-use-an-ex...
The likelihood that PHP is being used is strongly anti-correlated with company quality.
Correlation != Causation.
I think the causal effect works more like this: People in the startup scene (and particularly in SV) are unlikely to pick PHP as the language of their choice -> Not many PHP companies started by experienced entrepreneurs -> Lower quality.
The fact that PHP is correlated with lower startup quality (whatever that means) has probably nothing to do with PHP itself. If you forced all the smart people to use PHP, their companies would probably work out just as well.
"Acquia… today announced that its co-founder Dries Buytaert was named CTO of the Year at the 2014 MassTLC Leadership Awards, hosted by the Massachusetts Technology Leadership Council. Acquia led all companies with four finalists at this year’s awards…" [1]
Those examples show that PHP can be used, in a core way, by great technology companies.
[1] http://www.acquia.com/about-us/newsroom/press-releases/dries...
Though one problem: people use a tiny percentage of successful companies to justify their mistakes. One ad company manager told me, "Well Doubleclick's UI was slow!" But the obvious retort, if I thought he'd listen: "You're not at Doubleclick, are you? No, your company's failing miserably.
Same with PHP: "Don't imagine you're Facebook. You'd be lucky to be Myspace." If your tradeoff analysis suggests PHP is the right choice, so be it. But comparing oneself to Facebook suggests commitment to irrationality and monkey-see-monkey-do.
It'd be more reasonable to evaluate PHP in terms of tool quality, but that's too much to ask for in the tech industry.
I fully agree with you. Facebook is also a heavy PHP user, and that's the biggest startup IPO of the last decade. PHP can definitely be used at a great company -- the data simply showed that the better the company, the less likely it was to be using PHP.