Could anyone explain me please how to make a site with module system using OOP. For example I have a index.php file which includes modules from /modules folder.

Can you give me a step-by-step example please.

I would suggest that you familiarize yourself with the Yii Framework (or others). Their website has many tutorials to guide you in using the framework.

It is not a practical undertaking, with the technology available with a number of OOP frameworks in the wild, to be coding something like this from scratch. You will learn a LOT more by using an existing framework.

Can you share video tutorial here? So it is too easy for me to learn it.