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.