How would I hook into `clear_auth_cookie` to return the user’s ID that’s currently being logged out?

Question

That action doesn’t pass that data:

function return_user_data_on_logout( $user ) {

Here, $user will alway be undefined. Additionally, you need to tell add_action how many parameters the function takes.

But..

do_action( 'clear_auth_cookie' );

No information is passed to begin with, that’s not how this particular event/action works.

So how do we get the current user being logged out? The answer is we remove the words “being logged out” from that question giving us a much easier question that is far more searchable:

How do we get the current user?

$user = wp_get_current_user();

It’s possible that this hook may be too early, and additionally it may not be the best hook to use.

For example, wp_logout is a much better hook to use as it says on the tin what it does

So:

add_action( 'wp_logout', function() {
    $user = wp_get_current_user();
    // ...
});