Using the Apache Alias directive with PHP-FPM served by Apache

I read a lot of random help pages trying to find how to do this, and I thought I'd share my findings here so maybe future people won't have to search as hard.

So you have some PHP code that needs to be shared across multiple domains. Maybe you're doing some SAAS, or have some shared libraries, or whatever. You want to share the directory using Alias in your Apache configuration, and you need the PHP to execute.

Real-world use case: this is how the SimpleSAMLphp documentation tells you to...

Using the Apache Alias directive with PHP-FPM served by Apache