List taxonomy terms assigned to a post in hierarchical view

Question

Been trying do this and still no luck. What I'm trying is to list taxonomy terms assigned to a post ordered by parent-child category relationship (hierarchical view).

Structure is something like this

Library = custom post type      
Year = custom taxonomy assigned to "library" 
    
- 2021 = parent term     
-- April = child term
-- May = child term
-- June = child term
-- July = child term

The desired output is:

2021, April, May, June, July

I tried:

$terms = get_the_terms( $post->ID, 'document_year' ); 
if ( $terms && ! is_wp_error( $terms ) ) : 
    $output = array(); 
    foreach ( $terms as $term ) {
        $output[] = $term->name;
    }
    echo '<span>' . implode ( ', ', $output ) . '</span>'; 
endif;

But this outputs an alphabetical order of terms.

Same with this:

echo ( get_the_term_list( $post->ID, 'category', '', ' / ') );

Any help please?

Answer 1

If you have a need to build up an actual structure of terms, this might be a useful method for you:

/**
 * Recursively sort an array of taxonomy terms hierarchically. Child categories will be
 * placed under a 'children' member of their parent term.
 * @param Array   $cats     taxonomy term objects to sort
 * @param Array   $into     result array to put them in
 * @param integer $parentId the current parent ID to put them in
 */
function sort_terms_hierarchically(Array &$cats, Array &$into, $parentId = 0)
{
    foreach ($cats as $i => $cat) {
        if ($cat->parent == $parentId) {
            $into[$cat->term_id] = $cat;
            unset($cats[$i]);
        }
    }

    foreach ($into as $topCat) {
        $topCat->children = array();
        sort_terms_hierarchically($cats, $topCat->children, $topCat->term_id);
    }
}

Usage is as follows:

$categories = get_terms('my_taxonomy_name', array('hide_empty' => false));
$categoryHierarchy = array();
sort_terms_hierarchically($categories, $categoryHierarchy);

var_dump($categoryHierarchy);