Cheap WordPress Auto Login From Email Link

I am creating a membership site where I am trying to login a particular user from a link sent to their email address.

I have a custom post typed called “Members”, where I have stored a unique key with the name “user_key”. After a particular process, the email is sent to that user with an auto login link.

An example link looks like the following

http://example.com/process/?key=DcP2K7cmBUrLVRjizs78RAuuoMGRFc6F6TMDm6E6

Process is the page which handles the login part. Here is the code used in that page.

get_header();

$key = $_GET['key'];
$args = array(
    'post_type' => 'member',
    'meta_value' => $key
);

$properties = new WP_Query($args);

if($properties->have_posts()) : 
    while($properties->have_posts()) : $properties->the_post();
        $author = $post->post_author;

        $user = get_user_by('ID', $author);
        $user_id = $user->ID;

        echo $user_id;

        wp_set_current_user($user_id, $loginusername);
        wp_set_auth_cookie($user_id);
        do_action('wp_login', $loginusername);

        wp_redirect( home_url() );
    endwhile;
endif;
wp_reset_query();

get_footer();

When I go to that particular link, I get the following error.

“Warning: Cannot modify header information – headers already sent”

From what I understand, I think it’s the “wp_set_auth_cookie” part which is done after the headers are loaded, which is creating that problem.

Anyone has any ideas how to fix it?