It's basically a Caddy plugin that works as a drop in replacement for php-fpm, created by https://news.ycombinator.com/threads?id=kdunglas

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.

What is the Go part? It's a PHP interpreter or is PHP still doing all the work?

Go is acting as the process manager for PHP (either keeping a pool of workers running, or directly invoking PHP via CGO), and as the HTTP/HTTPS server (Caddy, obviously), ACME automation, etc. It's not a "PHP interpreter", it's literally PHP itself (written in C, compiled in with CGO).

The web server (Caddy) is written in Go.