I wouldn't really call it a drop-in replacement for php-fpm. It's more like emulating the way Apache+PHP was run traditionally, i.e. PHP is closer to the webserver instead of being a separate piece as it is with php-fpm (and needing fastcgi as protocol glue).
But yes, from a practical standpoint, your PHP apps should _just work_ if you run it with this instead of php-fpm, provided that you aren't doing anything weird.
If you're using a modern PHP framework (e.g. Laravel, Symfony) then you can use FrankenPHP's worker mode which is a huge boost in performance by keeping workers running which have the framework already warmed up and ready to accept requests right away, instead of having to boot the framework on every request.
But yes, from a practical standpoint, your PHP apps should _just work_ if you run it with this instead of php-fpm, provided that you aren't doing anything weird.
If you're using a modern PHP framework (e.g. Laravel, Symfony) then you can use FrankenPHP's worker mode which is a huge boost in performance by keeping workers running which have the framework already warmed up and ready to accept requests right away, instead of having to boot the framework on every request.