Form AJAX calling an external PHP file

Hi all!

I'm developing a custom module to get drupal working with other CMS.

I have a custom form displayed in a block. When the form is submitted the action param call a external PHP file that render an HTML page. Its all working fine.

Now I need to do the same process but in ajax mode. The html page must not be full rendered, I would like to get it and put into a div tag or similar solution. It will be a good solution if it's not displayed.

I tryed with the #ajax in the form definition i can not call an external PHP file using this. How can I use it? anyone knows an alternative solution?

thanks a lot!