It's likely that PHP is not so popular around here because most people that use Construct are interested in developing games without traditional coding. If you do see any talk about coding it will likely be Javascript and still in the context of the frontend.
Having said that, if you already are familiar with PHP and MySQL, there shouldn't be any problem in using that for your backend stack, it's a reasonable choice.
Despite being an absolute meme, PHP is a perfectly serviceable language!