EU VAT Assistant - Set VAT number required for some category

/**
 * Aelia EU VAT Assistant for WooCommerce - Make VAT number required when one
 * or more product from specific categories are in the cart.
 *
 * HOW TO USE THIS CODE
 * Add the code to the bottom of your theme's functions.php file (see https://www.skyverge.com/blog/add-custom-code-to-wordpress/).
 * The code will intercept the checkout process automatically and set the VAT number
 * field as "required", depending on the content of the cart.
 *
 * GPL DISCLAIMER
 * Because this code program is free of charge, there is no warranty for it, to the extent permitted by applicable law.
 * Except when otherwise stated in writing the copyright holders and/or other parties provide the program "as is"
 * without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of
 * merchantability and fitness for a particular purpose. The entire risk as to the quality and performance of the program
 * is with you. should the program prove defective, you assume the cost of all necessary servicing, repair or correction.
 *
 * Need a consultation, or assistance to customise this code? Find us on Codeable: https://aelia.co/hire_us
 */

/**
 * Returns the categories assigned to a product.
 *
 * @param WC_Product product
 * @return array
 */
function aelia_get_product_categories($product) {
  $result = array();
  // Get all the categories to which a product is assigned
  $categories = wp_get_post_terms($product->get_id(), 'product_cat');

  // The $categories array contains a list of objects. Most likely, we would
  // like to have categorys slug as a keys, and their names as values. The
  // wp_list_pluck() function is perfect for this
  $categories = wp_list_pluck($categories, 'name', 'slug');
  return $categories;
}

/**
 * Makes the VAT number required when products from specific categories are
 * added to the cart.
 *
 * @param bool vat_number_required
 * @param string customer_country
 * @return bool
 */
add_filter('wc_aelia_euva_order_is_eu_vat_number_required', function($vat_number_required, $customer_country) {
  // If the VAT number is already required, just return the flag as it is
  if($vat_number_required) {
    return $vat_number_required;
  }

  // An array of category IDs for which the VAT number is required
  $categories_requiring_vat_number = array(
    'category1',
    'category2',
  );

  foreach(WC()->cart->get_cart() as $item) {
    // $item['data'] contains a product instance
    if(!empty($item['data'])) {
      // The keys of the returned array are the category IDs
      $product_categories = array_keys(aelia_get_product_categories($item['data']));
      $matching_categories = array_intersect($categories_requiring_vat_number, $product_categories);

      // As soon as we find one product from one of the "VAT number required"
      // categories, we can stop
      if(!empty($matching_categories)) {
        $vat_number_required = true;
      }
    }
  }

  return $vat_number_required;
}, 10, 2);