Display only deepest category on a single post?

Question

I want to display the direct category ancestor of a given post. An illustrative example:

These are the categories I have:

• Cat1
• Cat2
  • Cat2.1
    • Cat2.1.1
    • Cat2.1.2
  • Cat2.2
    • Cat2.2.1
    • Cat2.2.2.
  • Cat2.3
• Cat3
• Cat4

I only put the posts in a single category between a same "level", but when the category has more sub categories, I check the whole trailing, like in:

• Cat1
• [X] Cat2
  • [X] Cat2.1
    • Cat2.1.1
    • [X] Cat2.1.2
  • Cat2.2
    • Cat2.2.1
    • Cat2.2.2.
  • Cat2.3
• Cat3
• Cat4

Now, in the single post page, I want to display the name of the direct category ancestor of the post (Cat2.1.2 in this case). By default I just use get_the_category(), but it shows the top level category instead (Cat2 in this case). I don't considerer unchecking top levels by now because it cause another problems within the template.

Answer 1

I come to this post as i was looking for the code. But the marked answer not working for me anymore. I have wrote a function to get the deepest level category assigned to a post.

function post_deepest_level_cat($post_categories) {
    foreach ($post_categories as $category) {
        $cat_ids[] = $category->term_id;
    }   
    $tree_args = array(
        'current_category' => $cat_ids,
        'depth'             => 50,
        'hierarchical'     => true,
        'echo' => 0,
        );                  
                
    $category_list = wp_list_categories($tree_args);                
    $dom = new DOMDocument;
    @$dom->loadHTML($category_list);
    $links = $dom->getElementsByTagName('a');
    $new_cat_array = array();
    foreach ($links as $link) { 
        $menu = get_term_by('name', $link->nodeValue, 'category');
        if (in_array($menu->term_id, $cat_ids)) {
            $deepest_cat_id = $menu->term_id;
        }                   
    }           
    return $deepest_cat_id;
}

And to use the above function here is the example.

$post_categories = wp_get_post_terms(10, 'category'); //All categories assigned to this post id "10"
$deeper_cat_id = post_deepest_level_cat($post_categories );
echo $deeper_cat_id; // will echo the id of deepest and last category of the current post.

This will help in 2020. If you have better code let me know. Thanks

Answer 2

It is possible that get_the_category() will always return the categories in the correct order (with the deepest at the end), but if it doesn't, you could also loop over all the categories and remove those where another category points to it as their parent.

$post_categories = get_the_category();
$categories_by_id = array();
foreach ( $post_categories as $category ) {
    $categories_by_id[$category->cat_ID] = $category;
}
foreach ( $post_categories as $category ) {
    unset( $categories_by_id[$category->category_parent] );
}
// $categories_by_id will now only contain the deepest categories

Answer 3

Well, I guess I found how (not completely tested):

<?php
$category = get_the_category($post->ID);
echo end($category)->cat_name;
?>

Basically, it returns the last category name from the array of categories given by get_the_category.