Global space

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.

<?php
namespace A\B\C;

/* This function is A\B\C\fopen */
function fopen() {
     /* ... */
     $f = \fopen(...); // call global fopen
     return $f;
}
?>