It's not a particularly elegant language.

Even with the RFC process, php-internals still resists adding language features useful only to advanced developers.

The standard library is full of warts, including odd spelling, inconsistent argument order, etc.

The killer though, as mentioned in the article, it has a low barrier to entry, so lots of neophytes write lots of bad code in it. There's also a tremendous amount of horrid, obsolete code out there, written by people that didn't know any better at the time, and kept up because the internet never forgets.

PHP is far less of a joke than it used to be, but people will continue to hate on it and look down on people that make a living writing it.