- PHP might be the most practical choice for some of these companies. The founder or lead tech might be most proficient in PHP for real-world applications (even if he adores Python or Ruby).
- PHP might be the language that everyone on the team knows so they choose it to keep everyone equally involved in the production of the site.
- The people commenting on HN aren't necessarily people who start companies (truer now than it ever was).
Edit: Also, it might be the case that some of these servers purposely lie about what language is used on the backend to thwart script kiddies.
Almost everyone on this site, including devoted PHP devs, recognizes the fact that PHP isn't exactly an elegant and well-designed language.
The pointless "hate" however only comes from a small group of people for reasons really only known to them. But if you look outside the world of programming languages to the kind of people that loudly proclaim to hate something, I think you'll get a pretty good idea of the kind of people we're talking about.
These are not the majority of users of HN. They just feel safe here, like hooligans feel safe in a stadium filled with supporters of the same team.
This list looks extremely noisy to me in general.
Not to defend the php hate, of course. Just to point out that it's not actually the most popular language, despite the misleading graph.
Bjarne Stroustrup
And second, I put my company's (admittedly not YC, but hey) homepage there just to try and since we're using a hosted Wordpress for the landing page so that business people can change the copy easier, it thought we're using PHP when in reality the app doesn't have a single line in it.
Additionally, Facebook has made PHP scalable with the HipHop compiler and other open source tools, so that's no longer a major reason not to use it.