Just a way to programmatically add hierarchical taxonomies in WordPress

wp-hierarchical-taxonomy.php

// For one level deep Wordpress nested terms.

$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
      )
    );
  }
}

// For three level deep Wordpress nested terms.

$taxonomy_term = 'example';
$related_terms = array(
    __( ' Parent 1 ' ) => array(
        __( ' Child 11 ' ),
        __( ' Child 12 ' ),
        __( ' Child 13' ),
    ),
    __( 'Parent 2') => array(
        __( ' Child parent 21' ) => array(
            __( ' Child 211 ' ),
            __( ' Child 212 ' ),
            __( ' Child 213' ),
            __( ' Child 214' ),
        ),
    ),

    __( ' Parent 3') => array(
        __( ' Child parent 31' ) => array(
            __( ' Child 311 ' ),
            __( ' Child 312 ' ),
            __( ' Child 313 ' ),
            __( ' Child 314 ' ),
            __( ' Child 315 ' ),
        ),
        __( ' Child parent 32 ' ) => array(
            __( ' Child 321 ' ),
            __( ' Child 322 ' ),
            __( ' Child 323 ' ),
            __( ' Child parent 324 ' ) => array(
                __( ' Child 3241 ' ),
                __( ' Child 3242 ' ),
            ),
        ),
    ),
);


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 $child => $term_second) {
        if (!is_array($term_second)) {
            $child = $term_second;
        }
        wp_insert_term(
            (string)$child,
            $taxonomy_term,
            array(
                'slug' => sanitize_title_with_dashes( (string)$child ),
                'parent'=> $term_id
            )
        );
        if (is_array($term_second)) {
            $parent_term_second = term_exists( $child, $taxonomy_term );
            $term_id_second = $parent_term_second['term_id'];
            foreach ($term_second as $child_second => $term_third) {
                if (!is_array($term_third)) {
                    $child_second = $term_third;
                }
                // print_r($term_child_second);
                wp_insert_term(
                    $child_second,
                    $taxonomy_term,
                    array(
                        'slug' => sanitize_title_with_dashes( (string)$child_second ),
                        'parent'=> $term_id_second
                    )
                );
                if (is_array($term_third)) {
                    $parent_term_third = term_exists( $child_second, $taxonomy_term );
                    $term_id_third = $parent_term_third['term_id'];
                    foreach ($term_third as $child_third) {
                        wp_insert_term(
                            $child_third,
                            $taxonomy_term,
                            array(
                                'slug' => sanitize_title_with_dashes( (string)$child_third ),
                                'parent'=> $term_id_third
                            )
                        );
                    }
                }
            }
        }
    }
}