What is the Scope Resolution Operator?

Answer:

"::" double colons is the scope operator it is used to call methods of a class that has not been instantiated. You should also understand static methods and how they differ from regular methods.

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