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

What does a `\` do in PHP? For example, [CSRF4PHP](https://github.com/foxbunny/CSRF4PHP/blob/60d9172b7f0cd93346cac9065fb17182854ebf1c/CsrfToken.php#L80-L87) 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); } }