If you use PHP the way it's meant to be used, you are not gonna have any surprise, and it'll run faster than the alternatives (or close too), for lower development time, as well as easiness in finding developers.
Also, the article is a bit off on some points, a website like Pornhub (100Million+ pageviews/day), is on the most standard stack you could imagine: PHP, Apache, MySQL, Memcached/Redis. Varnish get mentioned a lot, but when I was working there (not so long ago), it was not in use, and as far as I know Youporn might be the only one relying on it right now.
If you know what you are doing with PHP, you will have no surprise, no performance issue, and maintenance will be trivial. But sadly I have to admit few PHP developers actually use PHP the way it should be.
I wouldn't be surprised if they're using PHP for what it was originally meant to do (add a thin layer of dynamic-ness to straight html) and precomputing all of the data it uses in something else.
I can think of some special cases where PHP would be better, especially in a porn site's case -- the most common clicks are front-page links and there are probably a bunch of common keywords and clicks to links off the first page of those searches, which means that caching whole pages is probably economical. As far as I know, both Perl and PHP are identically suited to talking with upstream caching proxies, but PHP might have felt more natural for day-to-day feature development.
I presume there's a lot of legacy code there.
For anyone with a handful of servers PHP is acceptable as the additional cost of 2-3 servers is not really that high.
What's it used for in the porn industry, do you know? I didn't know there'd much need for IMing