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.
Exemple 20.5. Using __NAMESPACE__
<?php
namespace A::B::C;
function foo() {
// do stuff
}
set_error_handler(__NAMESPACE__ . "::foo");
?>