Wordpress auto login from custom page

In my wordpress website I have created a custom registration page to register in Wordpress using "PHP Code" plugin. Now I wanted to auto login the users when they complete the registration. I did lot of research but couldn't find a solution for my problem. I tried the following sample snippet to test in the custom page :

<?php

//Log in a WordPress user programmatically
function auto_login( $user ) {
    $username   = $user;
    if ( !is_user_logged_in() ) {
        $user = get_user_by('login', $username );
        $user_id = $user->ID;
        wp_set_current_user( $user_id, $user_login );
        wp_set_auth_cookie( $user_id );
        do_action( 'wp_login', $user_login );
    }     
}

auto_login('admin');

?>

But I am getting following errors.

Warning: Cannot modify header information - headers already sent by (output started at /home/content/19/11054119/html/wp-content/plugins/sitepress-multilingual-cms/inc/language-switcher.php:921) in /home/content/19/11054119/html/wp-includes/pluggable.php on line 925

Warning: Cannot modify header information - headers already sent by (output started at /home/content/19/11054119/html/wp-content/plugins/sitepress-multilingual-cms/inc/language-switcher.php:921) in /home/content/19/11054119/html/wp-includes/pluggable.php on line 926

Warning: Cannot modify header information - headers already sent by (output started at /home/content/19/11054119/html/wp-content/plugins/sitepress-multilingual-cms/inc/language-switcher.php:921) in /home/content/19/11054119/html/wp-includes/pluggable.php on line 927

So I am wondering is there anyway to do auto login without touching the theme functions. If you have any idea or solution help me with this..

Thanks