Wordpress: wp_list_categories -> Generate the current-cat class when viewing single posts.

wp-list-current-category.php

/* @Workether ------------------------------------------------------------
Generate the current-cat class when viewing single posts.
--------------------------------------------------------------------------*/
class singlePostCurrentCat {
  function wp_list_categories ($text) {
    global $post;
        $categories = wp_get_post_categories($post->ID);
        foreach ($categories as $category_id) {
          $category = get_category($category_id);
          $text = preg_replace(
            "/class=\"(.*)\"><a ([^<>]*)>$category->name<\/a>/",
            ' class="$1 current-cat"><a $2>' . $category->name . '</a>',
          $text);
        }
    return $text;
  }
}
add_filter('wp_list_categories', array('singlePostCurrentCat','wp_list_categories'));