$handler FQCN of handler. * * @throws InvalidArgumentException If the tag name is not a string. * @throws InvalidArgumentException If the tag name is namespaced (contains backslashes) but * does not start with a backslash. * @throws InvalidArgumentException If the handler is not a string. * @throws InvalidArgumentException If the handler is not an existing class. * @throws InvalidArgumentException If the handler does not implement the {@see Tag} interface. */ public function registerTagHandler(string $tagName, string $handler) : void; }