Sync taxonomy categories with ACF taxonomy field in WordPress

sync-parent-categories-acf.php

<?php
add_action('save_post', array($this, 'mark_parent_categories'), 11);

/**
 * Marks parent categories automatically. Works with the ACF taxonomy selector as well.
 *
 * @return mixed
 */
function mark_parent_categories($post_id) {
  global $post;

  if(isset($post) && $post->post_type !== 'page')
    return $post_id;

  //Set up ACF parent term array
  $acf_terms = array();

  // get all assigned terms
  $terms = wp_get_post_terms($post_id, 'page_category' );

  //For each term
  foreach($terms as $term) {

    $acf_terms[] = $term->term_id;

    //While not top level term
    while($term->parent != 0 && !has_term( $term->parent, 'page_category', $post )) {

      // move upward until we get to 0 level terms
      wp_set_object_terms($post_id, array($term->parent), 'page_category', true);

      //Add to acf term array
      $acf_terms[] = $term->parent;

      $term = get_term($term->parent, 'page_category');
    }
  }

  //Update ACF terms
  update_field('page_category', array_unique($acf_terms), $post_id);

  return $post_id;
}