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; } ?>
PHP General