no-framework-tutorial VS Tutorial-First-Application

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.

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