Should I use add_action(‘publish_post or add_filter(‘publish_post?
publish_post doesn’t work for custom post types, the correct hook (action hook) is publish_{$custom_post_type}. You should use add_action() as this is an action hook. I also tend to make use of the transition_post_status hook which is a much more universal hook as it fires everytime a post’s status is changed regardless. You can use $old_status … Read more