Answer 1

The double colon `::` is known as scope resolution operator. The other one `->` is member access operator (forgot its real title).

Both of these operator are used in relation to accessing a class' or object's members. So the difference in use cases is ...

:: operator
Used to access static members (properties or methods), or some would refer to as class members.

-> operator
Used to access non-static members (properties or methods), or some would refer to as object members.

Some relevant links 👇

https://www.php.net/manual/en/language.oop5.paamayim-nekudotayim.php

https://stackoverflow.com/questions/3037526/where-do-we-use-the-object-operator-in-php