::, Scope Resolution Operator, in Php

The Scope Resolution Operator (also called Paamayim Nekudotayim) or in
simpler terms, the double colon, is a token that allows access to
static,
constant, and overridden
members or methods of a class.

When referencing these items from outside the class definition, use
the name of the class.

Example #1 :: from outside the class definition


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

$classname = 'MyClass';
echo $classname::CONST_VALUE; // As of PHP 5.3.0

echo MyClass::CONST_VALUE;
?>



And, Static properties cannot be accessed through the object using the arrow
operator ->.

http://uk2.php.net/language.oop5.static