Without any namespace definition, all class and function definitions are
placed into the global space - as it was in PHP before namespaces were
supported. Prefixing a name with :: will specify that
the name is required from the global space even in the context of the
namespace.
Example #1 Using global space specification
<?php namespace A::B::C;
/* This function is A::B::C::fopen */ function fopen() { /* ... */ $f = ::fopen(...); // call global fopen return $f; } ?>