Wordpress. Login user after registration. (After a user has successfully registered, login automatically)

functions.php

// add this to functions.php file

function reg_new_user( $user_id ) {

    $userdata= get_userdata($user_id); // get new user data by ID

    wp_set_current_user( $user_id, $userdata->user_email ); //set user with ID and E-mail
    wp_set_auth_cookie( $user_id ); // set cookie by user ID
    do_action( 'wp_login', $userdata->user_email ); // wp_login action hook with user E-mail

}

add_action('register_new_user', 'reg_new_user');

// Tested in Wordpress 4.8