Scope Resolution Operator (::)
http://www.php.net/manual/en/language.oop5.paamayim-nekudotayim.php
The Scope Resolution Operator (also called Paamayim Nekudotayim) or in simpler terms, the double colon
“As of PHP 5.3.0, it’s possible to reference the class using a variable. The variable’s value can not be a keyword (e.g. self, parent and static). ”
Static Keyword
http://php.net/manual/en/language.oop5.static.php
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