PHP is one of the most accessible languages. If you are a web hosting service then you almost always support PHP - it's usually installed by a control panel, with loads of extras compiled into it. So, it's good if you're looking for a cheap hosting deal.
Magazines and tutorials teach the language by default. There's loads of competing free products to easily set up a development environment on most platforms. Which means lots of people who can build with it, and therefore, lower salaries.
Depending on the definition of "job", there might be many PHP jobs out there because WordPress gigs are typically shortlived deliverables (plugins, installs, etc.).
I actually stripped wordpress out as a skillset as I wasn't 100% certain I could match it to PHP programming skills. It did crop up a fair amount though, along with drupal and there's certainly a lot of those jobs about.
no idea I'm afraid, a whole lot of stufff is built in it. Glad you find it interesting, I'll be updating the stats each month so maybe we can see if PHP is on a down trend or just holding the top spot solidly.
Language accessibility is certainly the big driver but another, maybe equally important factor is availability. I do a fair bit of work for my brother who deals with low-end hosted accounts. All of these servers have PHP available where it would be impossible to get a Java installation or any other complex deployment solution in. PHP is everywhere.
Interesting list, thanks for sharing.