Twig_Environment::registerUndefinedFilterCallback PHP Method

    public function registerUndefinedFilterCallback($callable)
    {
        $this->filterCallbacks[] = $callable;
    }

 /**
  * Twig initialization that sets the twig loader chain, then the environment, then extensions
  * and also the base set of twig vars
  */
 public function init()
 {
     if (!isset($this->twig)) {
         /** @var Config $config */
         $config = $this->grav['config'];
         /** @var UniformResourceLocator $locator */
         $locator = $this->grav['locator'];
         $debugger = $this->grav['debugger'];
         $this->twig_paths = $locator->findResources('theme://templates');
         $this->grav->fireEvent('onTwigTemplatePaths');
         $this->loader = new \Twig_Loader_Filesystem($this->twig_paths);
         $this->loaderArray = new \Twig_Loader_Array(array());
         $loader_chain = new \Twig_Loader_Chain(array($this->loaderArray, $this->loader));
         $params = $config->get('system.twig');
         if (!empty($params['cache'])) {
             $params['cache'] = $locator->findResource('cache://twig', true, true);
         }
         $this->twig = new \Twig_Environment($loader_chain, $params);
         if ($debugger->enabled() && $config->get('system.debugger.twig')) {
             $this->twig = new \DebugBar\Bridge\Twig\TraceableTwigEnvironment($this->twig);
             $collector = new \DebugBar\Bridge\Twig\TwigCollector($this->twig);
             $debugger->addCollector($collector);
         }
         if ($config->get('system.twig.undefined_functions')) {
             $this->twig->registerUndefinedFunctionCallback(function ($name) {
                 if (function_exists($name)) {
                     return new \Twig_Function_Function($name);
                 }
                 return new \Twig_Function_Function(function () {
                 });
             });
         }
         if ($config->get('system.twig.undefined_filters')) {
             $this->twig->registerUndefinedFilterCallback(function ($name) {
                 if (function_exists($name)) {
                     return new \Twig_Filter_Function($name);
                 }
                 return new \Twig_Filter_Function(function () {
                 });
             });
         }
         $this->grav->fireEvent('onTwigInitialized');
         // set default date format if set in config
         if ($config->get('system.pages.dateformat.long')) {
             $this->twig->getExtension('core')->setDateFormat($config->get('system.pages.dateformat.long'));
         }
         // enable the debug extension if required
         if ($config->get('system.twig.debug')) {
             $this->twig->addExtension(new \Twig_Extension_Debug());
         }
         $this->twig->addExtension(new TwigExtension());
         $this->grav->fireEvent('onTwigExtensions');
         // Set some standard variables for twig
         $this->twig_vars = array('grav' => $this->grav, 'config' => $config, 'uri' => $this->grav['uri'], 'base_dir' => rtrim(ROOT_DIR, '/'), 'base_url' => $this->grav['base_url'], 'base_url_absolute' => $this->grav['base_url_absolute'], 'base_url_relative' => $this->grav['base_url_relative'], 'theme_dir' => $locator->findResource('theme://'), 'theme_url' => $this->grav['base_url'] . '/' . $locator->findResource('theme://', false), 'site' => $config->get('site'), 'assets' => $this->grav['assets'], 'taxonomy' => $this->grav['taxonomy'], 'browser' => $this->grav['browser']);
     }
 }