I have fordland.org, athletics.fordland.org, high.fordland.org, middle.fordland.org, and elementary.fordland.org. That is a total of 5 unique installations of php-Fusion, and it would certainly help if I could create a "cdn.fordland.org" where all the static files common across all 5 installations would be loaded from (rather than 5 different times as a user browses the entire district setup).
If you will go to apache.org there is documentation you will get all the details about URL re-writing with .htaccess.
But I think you will also have to get some change done to the database of php-fusion so that for every data it identifies for which subdomain it is and then based on it it is shown to the users. As I understand accomplishing what you need there has to be just one installation of php-fusion and once database backing it.