Global space
Name resolution rules
Namespaces
PHP Manual

## __NAMESPACE__

The compile-time constant `__NAMESPACE__` is defined to the name of the current namespace. Outside namespace this constant has the value of empty string. This constant is useful when one needs to compose full name for local namespaced names.

Example #1 Using __NAMESPACE__

`<?php
namespace A::B::C;
function foo() {
// do stuff
}
set_error_handler(__NAMESPACE__ . "::foo");
?>`

Global space
Name resolution rules
Namespaces
PHP Manual