Twig_Environment::registerUndefinedFunctionCallback PHP Method

    public function registerUndefinedFunctionCallback($callable)
    {
        $this->functionCallbacks[] = $callable;
    }

 public function run($viewContent, array $options = array())
 {
     $extendsBlock = '{% extends "' . self::DEFAULT_LAYOUT . '" %}';
     $contentStartBlock = '{% block content %}';
     $contentEndBlock = '{% endblock %}';
     $viewContent = $extendsBlock . $contentStartBlock . $viewContent . $contentEndBlock;
     if (array_key_exists('layout', $options)) {
         $layoutFileName = $options['layout'];
     } else {
         $layoutFileName = self::DEFAULT_LAYOUT;
     }
     $layoutFilePath = Application::getBasePath() . DIRECTORY_SEPARATOR . Application::getConfigItem('viewsFolder') . DIRECTORY_SEPARATOR . $layoutFileName;
     if (!file_exists($layoutFilePath)) {
         throw new \Exception('layout file could not be found');
     }
     $layoutContent = file_get_contents($layoutFilePath);
     $this->twigLoaderArray = new \Twig_Loader_Array(['layout.html' => $layoutContent, 'view.html' => $viewContent]);
     $loader = new \Twig_Loader_Chain([$this->twigLoaderFileSystem, $this->twigLoaderArray]);
     $twig = new \Twig_Environment($loader);
     $twig->registerUndefinedFunctionCallback(function ($name) {
         if (function_exists($name)) {
             return new \Twig_SimpleFunction($name, function () use($name) {
                 return call_user_func_array($name, func_get_args());
             });
             return false;
         }
     });
     echo $twig->render('view.html', $options);
     return;
 }