$publisher is outside the scope of the function adsense_for_feeds, and should therfore be declared as global.
ie.:
$publisher = 'ca-pub-XXXXXXX';
add_filter('the_content', 'adsense_for_feeds');
function adsense_for_feeds( $content ) {
global $post, $id;
$blog_key = substr( md5( get_bloginfo('url') ), 0, 16 );
if ( ! is_feed() ) return $content;
the line global $port, $id; should be changed to global $id, $post, $publisher