Each PHP file is an endpoint. As opposed to having routers in code or client side SPA routing.
PHP files can be deployed independently, swapped out or updated live.
No building/compiling of the php files needed.
A single layer as opposed to 'modern architecture' where there's client side back/front end layers, api layer, logic, validator, data access, and ORM layers.
Can extend itself as it runs. For example Wordpress, running off of php files can download plugins to its own server (which are just more php files) to instantly extend itself. Without restarting or redeployment. (What other web platforms can do this?)
Intuitive, simple, powerful. Can be as easy as editing a php file in notepad and dropping it on a ftp server. Deployed.
Amazon lambda may have more in common with PHP in terms of discreet deployable units of functionality. What's old is new again.
Compiling/deploying an entire system to change a single endpoint feels backwards after using PHP.