What does a \ (backslash) in front of function names mean?

What does a \ do in PHP?

For example, CSRF4PHP has \FALSE, \session_id, and \Exception:

public function __construct($timeout=300, $acceptGet=\FALSE){
    $this->timeout = $timeout;
    if (\session_id()) {
        $this->acceptGet = (bool) $acceptGet;
    } else {
        throw new \Exception('Could not find session id', 1);
    }
}