RenderToHtml视图文件并返回字符串?

我制作了一个小型的MVC框架,但是我需要我的view类能够以字符串的形式返回页面,但是在这之前应用所有的变量。怎么能做到?

class View{
    private $registry;
    private $vars = array();
    private $file;
    public function __set($index, $value){
        $this->vars[$index] = $value;
    }

    public function  __construct($controller, $action){
        $this->file = "Views/".$controller."/".$action.".php";
        $this->registry = new Registry();
    }

  public function renderToHtml(){
        try{
            if(file_exists($this->file)){

                foreach ($this->vars as $key => $value){
                   $$key = $value;
                }

                /** include $this->file; **/ 
//apply vars on file 
                return $html;
            }else{
                throw new Exception("No such View file");
            }
        }catch (Exception $e) {
            echo '<h1>Caught exception: ',  $e->getMessage(), "</h1>";
            exit;
        }
    }

控制器:

public function indexAction(){
   $html = new View('SomeController', 'htmlview');
   $html->somevar = "blabla";
   $this->View->fubar = $html->renderToHtml();
   $this->View->render() //this will render the current action/view (index) 
}

所以htmlview将是正则indexView中的一部分