Display selected category on post page

I know this is an old question, but I figured that I'd still weigh in on it.

I think I would do it with a get-request.

Below-written code has not been tested.

So in your category.php-file, then add the id of of the category, that the visitor came from, as such:

$category = get_the_category();
$link_to_the_post = '<a href="' . the_permalink() . '?cat_id=' . $category['ID'] . '">Link to post</a>';

And then check for that parameter in the single.php, as such:

if( !empty( $_GET['cat_id'] ){
  $category = get_the_category( $_GET['cat_id'] );
  echo 'Category: ' . $category['name'];
} else {
  // Fallback, showing all categories (in case the visitor came from some other place)
  the_category(); 
}

To get only the category through which you reached the post, you might use the query vars:

$category_name = get_query_var( 'category_name', 'No Category' );
echo ('Category: ' . $category_name);

The 2nd argument for get_query_var is the default value that is used when no category is returned.

Hope it helps!

I personally like to take all the categories a post is attached to and slap an active class onto them. Here's the code I found and currently use.

/** Active Category **/
// Single Posts only
function singlePostActiveCat ($CatText) {
 global $post;
 if (is_singular()) {
   $categories = wp_get_post_categories($post->ID);
   foreach ($categories as $category_id) {
    $category = get_category($category_id);
    $CatText = preg_replace(
       "/class=\"(.*)\"><a ([^<>]*)>$category->name<\/a>/",
       ' class="$1 current-cat"><a $2>' . $category->name . '</a>',
    $CatText);
    }
 }
return $CatText;
}
add_filter('wp_list_categories', 'singlePostActiveCat');