no-framework-tutorial VS adr-example

I myself learned by following the great "no-framework-tutorial": https://github.com/PatrickLouys/no-framework-tutorial Disclaimer: The last one is about 7 years old, so some things have changed, but all in all the information there is still valid.

No framework tutorial

Here's an all-in-one github repo that I found to be a nice build for the fundamentals: PatrickLouys/no-framework-tutorial

Use libraries or a micro framework if you are hesitant on using a full framework. Nothing wrong with Slim and some other libraries. Or bundle some libraries together to make your own slim (pun intended) framework - like this or this.

https://github.com/PatrickLouys/no-framework-tutorial also got turned into a book.

This tutorial (and subsequently the book) helped me a lot when learning OOPHP.

Do you already know how backends work? Maybe this can be a good read. Otherwise just look up the basic syntax and some quirks and you will be fine.

Search for "PHP without a framework" to find articles such as https://github.com/PatrickLouys/no-framework-tutorial that will show you how to "fill in the blanks" with libraries or simple code, or use a "microframework" such as Slim.

Adding to the above link a full ADR Example - https://github.com/pmjones/adr-example

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.

ADR (as opposed to MVC) - direct link - example

https://github.com/pmjones/adr-example - Follow up example to the above. ADR is an alternative to MVC

More on ADR (like MVC) - https://github.com/pmjones/adr-example

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

Simple example of a MVC/ADR (Action-Domain-Responder, similar to MVC) system - https://github.com/pmjones/adr-example

Call your class (which doesn't look like a controller anymore...), then, output it to your HTML or JSON renderer, based on the check. You can do this in a class (or 2 - look up ADR - https://github.com/pmjones/adr-example) or in the router.