How to print default category of a post in WordPress

Sometimes you need to print one category that characterizes the post. Imagine you have a “Most popular” widget and you wish to print the category link near each post title. How would you do that? Here is a short recipe. It applies to Wordress 3.4.1 but I assume it could be applied to a wide variety of versions.

1. Install the Hikari Category Permalink plugin. It adds the “Permalink” button near each category when you edit a post. Now you can define manually which category to treat as “main”
2. Write a small function anywhere you wish. I usually create a plugin named after a site name that  (example.com) and put all my helpers there. The function is like that:

   function example_get_main_category($post_id) {
       // extract the category from hikari permalink plugin
       $category_id = get_post_meta($post_id, '_category_permalink', true);
       if ( $category_id ) {
         return get_term($category_id, 'category');
       }
       // no main category defined, get the default one
       $terms = wp_get_object_terms($post_id, 'category');
       if ( $terms ) {
         return $terms[0];
       }
   }

   Note: By default WordPress would return the category with the lowest term_taxonomy_id.

3. Now you can output taxonomy link like this:

               $category = example_get_main_category($r->ID);
               $render_category = sprintf('<a href="%s" class="category-url">%s</a>', get_term_link($category), $category->name);
   

That’s all.