For all logged in visitors of my WordPress website I would like to set 2 cookies based on their user ID - so that I can use their user IDs in WordPress-unrelated PHP scripts at the same domain.

In one cookie I would like to store the user ID.

In the other cookie I'd like to store a md5 hash of the user ID and a secret word - so that I can verify in my PHP scripts, that the first cookie hasn't been faked.

Here is my code:

    $year = time() + 60 * 60 * 24 * 30 * 12;
    $auth = md5(join('_', array($user->ID, 'my_secret')));

    setcookie('visitor_id', $user->ID, $year, '/', '.example.com');
    setcookie('visitor_auth', $auth, $year, '/', '.example.com');

Please advise me, where to put this code in my plugin?

I have looked at the wp_set_auth_cookie, but I don't see how it could be used for my purpose. It seems to be a function to set some WordPress internal cookies, but not my custom cookies.

Or should I mayber put the above code into init hook? How can I find the user ID in there?