code: public function render($filter = null) : string
      {
          // Is output empty ?
          if (empty($this->output)) {
  
              // Extract variables as references
              extract(array_merge($this->vars, View::$global_vars), EXTR_REFS);
  
              // Turn on output buffering
              ob_start();
  
              // Include view file
              include($this->view_file);
  
              // Output...
              $this->output = ob_get_clean();
          }
  
          // Filter output ?
          if ($filter !== null) {
              $this->output = call_user_func($filter, $this->output);
          }
  
          // Return output
          return $this->output;
      }
docs: Include the view file and extracts the view variables before returning the generated output.
  
  // Get view
  $output = $view->render();
  
  // Display output
  echo $output;
  
  @param  string $filter Callback function used to filter output
  @return string