Recently, I was skimming through the WordPress Plugin Directory. I was looking for a plugin to hide a single category in WordPress, but doesn’t hide its posts. That plugin should only the category from the category listing beneath the (single) post and the sidebar. The reason is: I want to create an additional category that is listed nowhere in the frontend, but is still accessible from the category-archive page. Got that? ;-) No? Here’s a use case:

Every now and then, I am posting images from my flickr-account. I would like to place them in the category flickr. But the category flickr should not be named underneath the post This article was posted in: Photography and flickr., it should only name the category Photography. Furthermore, i do not want the category flickr to be listed in the sidebar. But I want all posts to be listed when accessing the link /category/flickr/.

Ok, this seems to be a pretty eccentric case, but it is just an example.

So I searched through the plugin directory and found … nothing. I’m not sure if the reason is in the search function itself, the plugin descriptions or my search phrases – I searched for exclude category, category exclusion, category visibility, etc. Eventually, I found nothing that I could use for my case. Most of the plugins I found only had a simple functionality: Choose one or more categories named X or Y and hide them completely. So they were not only hidden in the sidebar, in the feeds and in the posts, but also hid all posts of that category.

I challenged the search engine of my choice and quickly came down to hacking WordPress’ the_category-function. Here, I would want to hook in and hide the corresponding category. I searched further and found a solution on how to exclude a single category from WordPress’ the_category function.

The code to hide a single category in WordPress

I started with removing the category-entry from my single post entries – basically all template files had a code to display the list of categories that this article was published in. I improved the original function with the usage of category IDs. I like using IDs better, because they’re supposed to be unique and static. Even after renaming a category multiple times, the category ID will always remain the same.

You’ll obtain the category ID from WordPress’ backend when hovering over the category entry in Posts → Categories or when copying the link to the category. The link will look something like /wp-admin/edit-tags.php?action=edit&taxonomy=category&tag_ID=4&post_type=post. The number following tag_id= is the category ID you are looking for.

In your functions.php you add the following filter (source code quoted from the origin source and improved by using category IDs):

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function the_category_filter($thelist,$separator=' ') {
	if(!defined('WP_ADMIN')) {
		//Category IDs to exclude
		$exclude = array(17);
 
		$exclude2 = array();
		foreach($exclude as $c) {
			$exclude2[] = get_cat_name($c);
		}
 
		$cats = explode($separator,$thelist);
		$newlist = array();
		foreach($cats as $cat) {
			$catname = trim(strip_tags($cat));
			if(!in_array($catname,$exclude2))
				$newlist[] = $cat;
		}
		return implode($separator,$newlist);
	} else {
		return $thelist;
	}
}
add_filter('the_category','the_category_filter', 10, 2);

In line 4 of this function, you’ll add the category IDs that you want to exclude. To have multiple categories excluded, simply enter them as comma-separated values, e.g. $exclude = array( 7, 17, 25 );. The add_filter-hook finally alters the output of the_category based on the function. With this code, the corresponding categories will not be shown in your single posts and archive pages anymore, or simply said: whererever your template is using the the_category-function.

Hide the category in the category widget

Unfortunately, WordPress’ built in category sidebar-widget will not be affected by the above function to hide a single category in WordPress. You’ll need another one to accomplish that. But altering the category widget is very well documented in the WordPress Codex. I also simply copy and pasted the mentioned code:

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function widget_categories_args_filter( $cat_args ) {
       $cat_list = array( 17 );
       if ( $cat_args['exclude'] ) {
               //TODO: write code to insert category ids into existing list
       } else {
               $cat_args['exclude'] = implode( ',', $cat_list);
       }
       return $cat_args;
}
add_filter( 'widget_categories_args', 'widget_categories_args_filter', 10, 1 );

The code actually looks incomplete due to the comment in the TRUE-condition in the if-clause, but it is fully functional anyways: it excludes the categories by ID in the sidebar category widget.

Implementation

You will have to add both functions into your theme’s functions.php to make them work. If you are using a standard public WordPress theme, I strongly suggest creating a child theme, since all changes are overwritten when the theme gets an update.

I’m happy to take your hints on improving, simplifying or just shortening the code to hide a single category in WordPress, since I’m not a WordPress pro-developer. I just took the functionality as found – and it works! :-)