ElggEntity::setPrivateSetting PHP Method

    public function setPrivateSetting($name, $value)
    {
        if ((int) $this->guid > 0) {
            return set_private_setting($this->getGUID(), $name, $value);
        } else {
            $this->temp_private_settings[$name] = $value;
            return true;
        }
    }

/**
 * Add tags to the sent tags of an entity
 *
 * @param ElggEntity $entity       the entity to add to
 * @param string[]   $sending_tags the newly sent tags
 *
 * @return bool
 */
function tag_tools_add_sent_tags(ElggEntity $entity, $sending_tags = [])
{
    if (!$entity instanceof \ElggEntity) {
        return false;
    }
    if (empty($sending_tags)) {
        // nothing to add
        return true;
    }
    if (!is_array($sending_tags)) {
        $sending_tags = [$sending_tags];
    }
    $sent_tags = $entity->getPrivateSetting('tag_tools:sent_tags');
    if (!empty($sent_tags)) {
        $sent_tags = json_decode($sent_tags, true);
    } else {
        $sent_tags = [];
    }
    // store all processed tags
    $processed_tags = array_merge($sent_tags, $sending_tags);
    $processed_tags = array_unique($processed_tags);
    return $entity->setPrivateSetting('tag_tools:sent_tags', json_encode($processed_tags));
}