您不知道传递给$user 参数，让我们测试一下。

您将希望详细介绍这些条件和响应。这只是您可能想要进行的测试的一个示例。

function user_registered( $user ) {
    // Check to make sure it\'s not an error.
    if ( is_wp_error( $user ) ) {
        return;
    }
    // This is how WordPress checks to make sure the user exists.
    // This could also apply to many other objects, though. 
    if ( ! isset( $user->ID ) ) {
        return; 
    }
    // This checks to make sure you\'re getting the expected user object fields.
    if ( ! isset( $user->user_email ) ) {
        // If you have a correct ID, you can still retrieve the user\'s fields.
        $user = get_user_by( \'ID\', $user->ID );
        // If the user doesn\'t exist, stop where you are.
        if ( ! $user ) {
            return false;
        }
    }

    $headers = array(
        \'Content-Type: text/html; charset=UTF-8\',
        \'From: WordPress Website <admin@example.com>\',
    );
    wp_mail( $user->user_email, \'Subject of email\', \'Email body\', $headers );
}