php 8.x compatible code

<?php
/*
Plugin Name: BAW Login/Logout menu
Plugin URI: http://www.boiteaweb.fr/?p=3337
Description: You can now add a correct login & logout link in your WP menus.
Author: Juliobox
Author URI: http://wp-rocket.me
Version: 1.3.3
*/

define("BAWLLM_VERSION", "1.3.3");

add_action("plugins_loaded", function () {
    $filename = "inc/";
    $filename .= is_admin() ? "backend-" : "frontend-";
    $filename .= defined("DOING_AJAX") && DOING_AJAX ? "" : "no";
    $filename .= "ajax.inc.php";
    if (file_exists(plugin_dir_path(__FILE__) . $filename)) {
        include plugin_dir_path(__FILE__) . $filename;
    }
    $filename = "inc/";
    $filename .= "bothend-";
    $filename .= defined("DOING_AJAX") && DOING_AJAX ? "" : "no";
    $filename .= "ajax.inc.php";
    if (file_exists(plugin_dir_path(__FILE__) . $filename)) {
        include plugin_dir_path(__FILE__) . $filename;
    }
});