question on category_save_pre function

I’m trying to write a function which manipulates certain categories prior to saving. I assume I should use the filter: category_save_pre

I would like to write a function which tests for a certain category and if it finds it I will save a different category under certain circumstances. I assume if I use:

add_filter(‘category_save_pre’,’sb_change_category’);

Then my function will be passed a coma-delimited list of ID #’s and I can return a coma-delimited list of ID #’s. However, it doesn’t appear to work. For example, if I simply write a function like this:

function sb_change_category($cat_holder)
{
$cat_holder = ‘3’;
return $cat_holder;
}

My posts will be saved as uncategorized. Of course I don’t want to just change all of my categories to category 3, but nothing seemed to work so I tried to see if I can get anything to work.

Any help will be appreciated.

Thanks