Question

I guess, it’s possible to call functions in a separate class, I call this object-oriented programming in PHP.

But at the beginning, there is always an index.php or something, which calls or instantiates another class.

Is there a way at all to have a class which is self-calling in a way like java does with

public static void main(String[] args) ...

Answer 1

Not by default. But you could write your own bootstrap that does this, see a related answer that outlines how you can do it.

If index.php is your only central entry point, it’s already the bootstrap.

From that point on, you can do everything OOP you like. However, I think your question is less about OOP but more about how you deal with the invokation of your application code.