Custom function ends in "Undefined function"-Exception

Hi community,

I am trying to define my own function but it lags at the very beginning. In my services.php I have my service "view" where I add the engine "Volt" and the custom function but in the view I try to call this function {{ tata("df") }} and there comes the exception.

$di->set('view', function() use ($config) {

    $view = new \Phalcon\Mvc\View();
    $view->setViewsDir($config->application->viewsDir);

    $view->registerEngines(array(
//        '.volt' => "voltService"
        '.volt' => function ($view, $di) use ($config) {

            $volt = new Phalcon\Mvc\View\Engine\Volt($view, $di);
            $volt->setOptions(array(
                'compiledPath'      => $config->application->cacheDir,
                'compiledSeparator' => '_'
            ));
// This does not work either
//            $compiler= $volt->getCompiler();
//            $compiler->addFunction("eike", function($str) {
//                return "xdebug_var_dump($str)";
//            });
                $volt->getCompiler()->addFunction(
                    'tata',
                    function ($key) {
                        return "xdebug_var_dump({$key})";
                    }
                );

            return $volt;
        }
    ));
    return $view;
}, true);

greetings Eike