I don't really understand your question, but you mean either one of two things:
You either want to generate a page with PHP and use the variable in your URL somewhere. You can use either the variable name ($formIndex), $HTTP_GET_VARS['forumIndex'] or $_GET['forumIndex'] depending on your PHP installation
You want the browser to automatically scroll down the page to a designated location. For this you would put an anchor at the location (<a name='yourAnchor'>), and add #yourAnchor to the end of the url (after all the variables too, the # sign is the delimiter).