invention-project VS no-framework-tutorial

If you mean the skeleton for creating new applications, then you're probably looking for https://github.com/onesoft-sudo/invention-project But If you're looking for 'examples' or docs, then it is currently unavailable.

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.