Display deepest post category, when post have multiple categories

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
  Cat3.1
   Cat3.1.1
 Cat4

I check the whole trailing, like in:

  Cat1
  [X]Cat2
    [X]Cat2.1
        Cat2.1.1
     [X]Cat2.1.2 <-my post
       Cat2.2
        Cat2.2.1
        Cat2.2.2
       Cat2.3
  [x]Cat3<
    [x]Cat3.1
     [x]Cat3.1.1 <-my post
     Cat4

So I have 'my post' assigned to a multiple categories. Lets say I am navigating to 'my post' like this : Cat3->Cat3.1->Cat3.1.1->my post. Now, in the single post page, I want to display the name of the direct category ancestor of 'my post' (Cat3.1.1 in this case). But the question is how to do this?

Answer 1

<?php
$ancestors_array = get_ancestors($category_id, 'category');
$very_first_ancestor_id = end($ancestors_array); // Cat3
$parent_id = reset($ancestors_array); // Cat3.1

See Function Reference/get_ancestors