Add or remove product to specific category based on product custom attributes

functions.php

/**
 * Add / Remove product to 'Angolo delle offerte' on product save action
**/
function update_product_category( $post_id ) {
  global $post;
  if(function_exists('get_product')) {
    $product = get_product($post->ID);
    if($product->is_type('variable')) {
      $available_variations = $product->get_available_variations();
      $catarray = array();
      $myarray = array();
        foreach($available_variations as $k) {
          $_in_promo = get_post_meta($k[variation_id], '_in_promo', true);
          $_special_price_to_date = get_post_meta($k[variation_id], '_special_price_to_date', true);
          $now = date("Y-m-d");
          $exp = date("Y-m-d", strtotime(str_replace('/', '-', $_special_price_to_date)));
          if(($_in_promo == yes) && ($now<=$exp)) {
                $myarray[k.$exp] = $_in_promo;
          }
        }
      $newarray = array_merge($catarray, $myarray);
      $newarray = array_filter($newarray);
      if(!empty($newarray)) {
        wp_set_object_terms($post->ID, 'angolo-delle-offerte', 'product_cat', $append = true);  
      } else {
        wp_remove_object_terms($post->ID, 'angolo-delle-offerte', 'product_cat' );
      }
    }
  }
}
add_action( 'save_post', 'update_product_category' );