Description:
------------
Hello,

currently it's not possible to see if PHP tries to fallback to a global function/constant when the function/constant isn't found in the current namespace.

This can lead to perfomence problems when developers convert older classes to PHP 5.3 namespaces and forget to prefix standard functions with a backslash.

This also helps people to learn what functions need to be called from the global space and which not.

Test script:
---------------
namespace Foo;

function bar() {
    // would be nice if that would trigger a warning
    return phpversion();
}

function baz() {
    // no warning
    return \phpversion();
}

Expected result:
----------------
a error level constant E_NAMESPACE_FALLBACK to use for error_reporting()