Allow a user to ‘subscribe’ to different post categories?

So this hasn’t been 100% tested yet, but the basic logic is there.
The IF checks for the presence of the catID and if it isn’t there; update user meta with current list + category ID.

Need to do some more work on removing from the list but it’s the same principal. Check string for category ID. If exists, remove from string and store as new string. Update user meta with that string.

global $current_user;
get_currentuserinfo();

$user_id = $current_user->ID;
$currentList = $current_user->subList;
$cat_id = get_query_var('cat');
$catStr = " " . $cat_id . ",";
$update;

if (strpos($currentList,$catStr) !== false) {
echo 'ALREADY IN LIST';
}else{
echo 'ADDED TO LIST';
$update = $currentList . $catStr;
update_user_meta($user_id, 'subList', $update);
}