I'd like to use a form param to personalize an RSS feed.

For example, when I grab something like:

http://www.domain.com/sample.rss?I=12345

I'd like the content to popular a the I= variable with I=12345

I know how to do this with PHP... I simple use the echo command:
I=<?php echo $I?>

Any thougths?

I presume you mean you would like to make a 'dynamic' RSS page. For
this you can use PHP, just return a content-type of
application/rss+xml so that the browser accessing the page knows what
kind of data it contains. You could also setup your webserver to
process .rss documents with php.