Check if post is being published for the first time, or is an already published post being updated

Question

I am building an add-on to an events module that checks for availability, since there wasn't a function for that in that module. Now that I have built the logic, there are three emails I will need to send:

• One if the post is being submitted for the first time (ie 'New appointment')

• One if the post has been published by the site admin, as normal users won't be able to publish by themselves (ie 'Your appointment has been approved')

• One for if the post has been edited, AFTER it has been published already (ie 'Your appointment has been edited')

I have hooked into save_post and publish_post for those first two, but I want an entirely different email sent upon editing a PUBLISHED post. How can I test for whether the post is already published and this is just an edit, versus it being published for the first time?

Answer 1

Hook into edit_post to catch changes. And take a look at wp_transition_post_status() which is called on inserts and updates:

function wp_transition_post_status($new_status, $old_status, $post) {
    do_action('transition_post_status', $new_status, $old_status, $post);
    do_action("{$old_status}_to_{$new_status}", $post);
    do_action("{$new_status}_{$post->post_type}", $post->ID, $post);
}

On publish you hook into

• draft_to_publish,
• pending_to_publish and
• auto-draft_to_publish.

For edits hook into publish_to_publish.