How to get current_cat in navbar, in single post

In a site I have using WordPress 3.0, when I’m in a single post, the navbar showing the categories doesn’t give the parent category the class of “current_cat” and therefore that category is not highlighted.

How can I make WordPress give the parent category that class in single_post mode?

I found the answer here.
Add to functions.php the following function and hook:

function sgr_show_current_cat_on_single($output) {

global $post;

if( is_single() ) {

    $categories = wp_get_post_categories($post->ID);

    foreach( $categories as $catid ) {
        $cat = get_category($catid);
        // Find cat-item-ID in the string
        if(preg_match('#cat-item-' . $cat->cat_ID . '#', $output)) {
            $output = str_replace('cat-item-'.$cat->cat_ID, 'cat-item-'.$cat->cat_ID . ' current-cat', $output);
        }
    }

}
return $output;
}

add_filter('wp_list_categories', 'sgr_show_current_cat_on_single');