Programmatically add hierarchical taxonomies in Wordpress

wp-hierarchical-taxonomy.php

$taxonomy_term = 'example';

$related_terms = array(
  __( 'Parent' ) => array(
    __( 'Child 1' ),
    __( 'Child 2' ),
  ),
  __( 'Parent 2') => array(
    __( 'Child 3' ),
    __( 'Child 4' ),
  ),
);

foreach ($related_terms as $key => $term) {

  wp_insert_term(
    (string)$key,
    $taxonomy_term,
    array(
      'slug' => sanitize_title_with_dashes((string)$key),
    )
  );

  $parent_term = term_exists( $key, $taxonomy_term );
  $term_id = $parent_term['term_id'];
  
  foreach ($term as $term_value) {
    wp_insert_term(
      $term_value,
      $taxonomy_term,
      array(
        'slug' => sanitize_title_with_dashes( $term_value ),
        'parent'=> $term_id
      )
    );
  }
}

With help of your code I was able to adapt it to also programmatically create Wordpress terms that are nested three levels deep.
I created a fork from this github page so that other people may also benefit from it.

https://gist.github.com/richardscholtens/b87ab3496e2460188fff26ae8fc3e27f