This is cool, it's a tool to transpile it at build time so you are two steps removed from running PHP on the browser. I dissected this tool a while back and built another that lets you write and run PHP straight in the browser by doing the transformation on-the-fly. Wrote a blog post and all around it:
https://francisco.io/blog/running-php-in-javascript/
Incidentally it's the #3 when you google "Run PHP in Javascript"