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?

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  1. 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');