As much as people hate MySQL and PHP stack. I think a large percent of the web is still using it. And it is comparatively cheaper. For example, even the beginner web developer can deploy it easily without having to use VPS.
However there are some changes coming these days with many PHP developers moving from MySQL to PostgreSQL. I am not sure what could be the reason. You can PHP PDO Connect to Postgresql easily. And there is less code changes to make if you are using PDO.
Startups can save a lot of money with the PHP and XSQL stacks. Many freelancers and low cost business are saving the money with that stack. I don't see this changing any time soon.