I think you underestimate the number of PHP developers out there. There aren't many on Hacker News because we're all too cool for that stuff, but in the developer world in general there are a ton.
I'd imagine people signing up to work at FB do so despite the fact they use PHP, not because of it.
In my experience, the top-tier developers enjoy challenges and solving problems, and languages are just tools they use to defeat those challenges and solve those problems. Sometimes it's a language they really enjoy using. Sometimes it's a situation where that language isn't a suitable one.
I imagine it's neither "despite" nor "because" of PHP use; any top-tier developer is going to use the tools available to him/her, rather than pick an employer in order to use a specific set of tools. Lower-quality developers may do so, but that's probably because they're less likely to know the right tool for the job (eg someone may hate functional programming and avoid jobs that use it, but that's because they don't know when FP is the ideal way to solve the problem)
But of course, anyone who's worked on production systems knows that all the mathematical ideals in the world doesn't stop bugfixes from becoming a giant hairy mess. It's more your culture, skill, and process that allow or prevent that from getting refactored into a better solution and how long that takes. Any language can be a part of a clean or messy codebase, although some tend to skew towards one side or another - but I'd wager that's as much a factor of barrier to entry as anything else.
Actual top-tier developers don't fret much about it. Complaining about some language's warts is mostly for bike-shedding and blogging types. PG, that writes about blub languages and who-cool-is-list, used Perl in ViaWeb too (which is as inelegant a mess as PHP is, but at the time was good for web work).
Top tier developers get shit done in whatever language. And most of them rarely ever comment or blog.