I have a standard page setup at `http://mysite.com/mypage/`

I'd like to setup a rewrite rule in my theme functions that takes any URLs such as:

`http://mysite.com/mypage/helloworld/`   <-- Currently shows "Not Found"

And translates it into:

`http://mysite.com/mypage/?var=helloworld`  <-- Currently shows the standard page

NOTE ADDED:

"helloworld" is an unknown variable value, so I want ALL "/mypage/" urls to rewrite back, for example:
`http://mysite.com/mypage/abc/`
`http://mysite.com/mypage/1231241/`
`http://mysite.com/mypage/sdfasfdadsf/`

Will all rewrite back to `http://mysite.com/mypage/?var={path}`