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.

I think PHP's popularity has a lot to do with WordPress's popularity. For example, near half of the top 10k websites use it: http://www.wptavern.com/wordpress-powering-practically-half-...

Depending on the definition of "job", there might be many PHP jobs out there because WordPress gigs are typically shortlived deliverables (plugins, installs, etc.).

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.