This function is declared in wp-admin/includes/taxonomy.php. This file will be loaded in wp-admin/includes/admin.php. And that now is called in wp-admin/admin.php. So the solution is: Use the hook ‘admin_init’ or require all necessary files manually. But don’t do that on every request.
WordPress Category Page Layout
Include specific category in URL automatically
Good for @Tom J Nowell♦ solution would be more properly. But it still could fulfill the goal which add the extra function in your class. class List_Categories_Radiobuttons extends Walker_Category { function start_el(&$output, $category, $depth = 0, $args = array(), $current_object_id = 0) { $category_name = esc_attr($category->name); $radiobutton = ‘<input class=”form-check-input” type=”checkbox” value=”‘ . $category_name . … Read more
Getting the posts list visualised in categories tree
Subcategory Page URL to Display Only Posts from that Subcategory?
How to display a block only for a certain category page
Create 2 wp_query and put pagination on the first wp_query and remove pagination on second wp_query
I think your logic is too complicated. Setting up query_posts() is essentially setting up MySQL query for database. No way query would be able to easily count amount of assigned categories on the fly and run conditions against that. Let’s flip your condition. Instead of posts that aren’t in slideshow category alone we can do … Read more
the_category() uses get_the_category_list() to do its work, but this function gives you no way to specify classes. However, you can filter the output with the the_category hook. Since you know the format of the current category links, you can so a search and replace on them. It would look something like this (untested): add_filter(‘the_category’, ‘highlight_current_cats’, … Read more