For instance:

html://www.mysite.com/subdom/index.php?abc=1&def=2&ghi=3

to look as:

html://wwww.mysite.com/subdom/index.php

or even:

html://wwww.mysite.com/subdom/

Create a page at whatever.com/index.php that's only an iframe that can take in the whole from the browser window. That iframe points at the actual index page, so when people click links it only changes the page the iframe reaches, not the outer page and also the URL within the location bar stays exactly the same.

You should use Apache's [cde] to rewrite your Web addresses.