src/Sonic/Routing/Router.php
public function not_found($callback = null)
    {
        if (is_callable($callback)) {
            // Set provided callback function as not found handler
            $this->not_found_handler = $callback;
        }
        else {
            // Set HTTP status on router
            $this->current_http_status = \Symfony\Component\HttpFoundation\Response::HTTP_NOT_FOUND;

            // Execute not found handler and set result as response content
            $result = is_callable($this->not_found_handler) === TRUE
                ? call_user_func($this->not_found_handler)
                : $this->default_not_found_handler()->execute();

            $this->response->setContent($result);

            // Set response's status code
            $this->response->setStatusCode($this->current_http_status);

            // Send response
            $this->response->send();
        }
    }
public function not_found($callback = null)
    {
        if (is_callable($callback)) {
            // Set provided callback function as not found handler
            $this->not_found_handler = $callback;
        }
        else {
            // Set HTTP status on router
            $this->current_http_status = \Symfony\Component\HttpFoundation\Response::HTTP_NOT_FOUND;

            // Execute not found handler and set result as response content
            $result = is_callable($this->not_found_handler) === TRUE
                ? call_user_func($this->not_found_handler)
                : $this->default_not_found_handler()->execute();

            $this->response->setContent($result);

            // Set response's status code
            $this->response->setStatusCode($this->current_http_status);

            // Send response
            $this->response->send();
        }
    }
This method can either set the callback to execute on a 'Page not found' (404)
error, or it invokes the callback
To set the callback, provide a function to the method
To invoke the callback, call the method without any parameters

@param  \Closure $callback
@return