The Scope Resolution Operator is also called Paamayim Nekudotayim or in other words the double colon, is a token that allows access to static, constant, and overridden properties or methods of a class.

Example:  

<?php
class Slightbook{
    const CONST_VALUE = 'A constant value';
}

$classname = 'Slightbook';
echo $classname::CONST_VALUE;

echo Slightbook::CONST_VALUE;
?>