>Everyone says it, but I don't think I've seen any solid arguments behind it

You haven't looked very hard then:

http://webonastick.com/php.html http://me.veekun.com/blog/2012/04/09/php-a-fractal-of-bad-de... http://www.ukuug.org/events/linux2002/papers/html/php/index.... https://maurus.net/resources/programming-languages/php/