You can log in a user to your WordPress programmatically without validation (username or password) by using these PHP codes:

$username = 'an username';
if (!is_user_logged_in()) {
    $user = get_userdatabylogin($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);
}