Tutorial-First-Application VS framework-x

b) Separate out all SQL and PHP database code. Put these in separate classes similar to this example Slim First Application Mapper class. You can decide later if you want to use a framework's DB code or not - it would be simply refactoring your existing code and not changing (hopefully) the API that you started using internally

Yes, as an example, the First Application's business logic (not that it's much) doesn't tie itself to the framework, as one example. Odan's Slim Skeleton, ADR Example (yes, I know you are the author), and the Aura Payload example class can be seen as other examples.

Both. Build business logic with PHP, then use a framework for the common functionality (routing, auth, validation, etc). Look at Slim's First Application to show a very simple example of this - https://github.com/slimphp/Tutorial-First-Application

Another option, to me and speaking to the OP as an alternative, it is simpler to add some Data Mappers/Repositories to abstract away the Database code. Good examples is the sample ADR application, Payload library example, Slim 3 First Application, or this Slim 4 Skeleton application

Yes, you can. Did you look at the example I provided? Want another one? Slim Framework's first application does exactly this:

https://github.com/slimphp/Tutorial-First-Application <- Uses the Slim framework

Have a look at Framework X (https://framework-x.org). It's barebones, but provides routing and basic request/response handling to build your own stuff upon. It's based on ReactPHP so you can use it as standalone service or use it in shared hosting with FPM, whichever suits you.

Using FrameworkX default hello world example I get 12.4K req/s on my M1. FrameworkX is using ReactPHP. Unless I'm reading activity monitor wrong it's ~10MB idle and 12,5 MB max.

There are many micro-frameworks available like Slim, Mezzio(with Swoole may be), or if you are into async then you can also try Framework X(from core maintainer of ReactPHP). This frameworks have small footprints compare to Laravel, Symfony, etc. Yes, you can't get things like authentication, routing and other out of the box but they can easily be installed and configured to be used.

I saw it was mentioned in the Framework X

As others have mentioned, in certain scenarios using a long-running application server (rather than mod_php/php-fpm) to serve requests can make sense. There are a number of extensions and libraries available that facilitate this with asynchronous code execution - Swoole has already been mentioned but you may also want to check out ReactPHP (not related to JS React, other than they stole the name) and Framework X

The Framework X is an attempt to make ReactPHP into a micro framework, but I see little mention of it.

Framework-x https://github.com/clue/framework-x

With ReactPHP, I've written a couple websocket apps. Worked pretty great for that. https://framework-x.org/ is also a pretty good place to get started for a simple HTTP microservice-type app.

Yeah ReactPHP will do this. I have an event source server running and it was pretty easy (once you get the hang of using react). I had an nginx proxy in front of it for tls. Have a look at https://framework-x.org too