It’s the backbone for a lot of other software, the Wordpress ecosystem most notably. Inertia is the big factor in terms of popularity.

Old school PHP was all about the LAMP stack (Linux as your operating system, apache as your web server, mysql as your database, php as the runtime apache would use to serve dynamic documents). The ecosystem was the real benefit that made it sensational for the era; all the pieces were designed to play nicely with one another.

It was also really noob friendly: You could write basic html, copy it to your web server, reload in browser, and see changes. Then to make things dynamic, you sprinkle in a few <?php ?> tags in your html, copy it to the server, and it starts working! The code inside those php tags gets run by the web server and their output is written into the HTML. No reloading or resetting apache needed, and you need very little programming knowledge to start.

The modern language is… serviceable, but lost the key features that made it great.