[TYPO3-mvc] php view problem

[hannes]

Hallo,
I want to use ajax in my current extbase fluid project. Therefore I 
defined a php view. I also
defined a new plugin which calls the ajax action by default, and defined 
a typenum in typoscript which
points to the plugin. The ajax action works with an html view, but not 
with an php view(ajax.php).

How can I tell a Controller action to call a php view instead of an html 
view?

ajax.php:
class Tx_EfectoProcastme_View_User_Ajax extends 
Tx_Extbase_MVC_View_AbstractView
{
    public function render(){
       
        return 'Hello World';
    }
   
}

Best Regards Hannes
I