Hook before user is created and make some custom validation

Question

I try to hook in user creation process in admin dashboard /wp/wp-admin/user-new.php to make some custom validation on data with the following hook, but the hook is not executed on form submit. What goes wrong in this case?

What I want, I have a custom ACF relation field which should be mandatory only if the selected role is editor. So I want to parse the request and check against the rules. If it fails than do not create the user return the custom error message.

add_action( 'register_post', function($user_login, $user_email, $errors) {
    var_dump('test');
    die;

    $userIsValid = ValidateUser::make($errors);

    if(!$userIsValid) {
        $errors->add( 'bad_email_domain', '<strong>ERROR</strong>: errors' );
    }
});

Answer 1

You need to add , 10, 3 after your closure to set the hook priority and (more importantly) the number of accepted arguments - otherwise $user_email and $errors will not be passed.

add_action( 'register_post', function ( $user_login, $user_email, $errors ) {
    $userIsValid = ValidateUser::make($errors);

    if(!$userIsValid) {
        $errors->add( 'bad_email_domain', '<strong>ERROR</strong>: errors' );
    }
}, 10, 3 );

Update: The register_post hook only fires inside register_new_user(), which is only used for user registration via the WordPress login page.

To handle errors within the admin (i.e. Users > Add New) use the user_profile_update_errors hook instead.