Websites redirects

Hello,

I've created a new virtual host NEWdomain.com to create a new web site
This domain is not registered yet.
To work on it and allow the owner to see previews I created an 'alias to directory' redirect
mydomain.com/NEWdomain -> /home/NEWdomain/public_html

Going to http://mydomain.com/NEWdomain my browser displays a Forbidden warning.
When I create an index.html the browser opens the page as expected but when I substitute index.html by index.php a window "You have chosen to open: (blank here)... which is a phtml file....Open? Save? ".

I guess this is a security issue about php-cgi and owners.

How can I handle that?

Thank you
M