This is the original index.php file - unchanged:
<?php
/**
* index.php -- This is the main controller file for the Zen Cart installer
* @copyright Copyright 2003-2020 Zen Cart Development Team
* @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
* @version $Id: Scott C Wilson 2019 Jul 16 Modified in v1.5.7 $
*/

// Actual version check is more strict; this is just to start the program
// For true minimum, see includes/systemChecks.yml under checkPhpVersionMin
if (PHP_VERSION_ID < 50500) {
die('Sorry, requires minimum PHP 5.5');
}

define('IS_ADMIN_FLAG',false);

/* Debugging
* 'silent': suppress all logging
* 'screen': display-to-screen and also to the /logs/ folder (synonyms: TRUE or 'TRUE' or 1)
* 'file': log-to-file-only (synonyms: anything other than above options)
*/
$debug_logging = 'file';

/*
* Ensure that the include_path can handle relative paths, before we try to load any files
*/
if (!strstr(ini_get('include_path'), '.')) ini_set('include_path', '.' . PATH_SEPARATOR . ini_get('include_path'));

/*
* Initialize system core components
*/
define('DIR_FS_INSTALL', __DIR__ . DIRECTORY_SEPARATOR);
define('DIR_FS_ROOT', dirname(__DIR__) . DIRECTORY_SEPARATOR);

require DIR_FS_INSTALL . 'includes/application_top.php';

if ($controller == 'cli') {
require DIR_FS_INSTALL . 'includes/cli_controller.php';
} else {
require DIR_FS_INSTALL . $page_directory . '/header_php.php';
require DIR_FS_INSTALL . DIR_WS_INSTALL_TEMPLATE . 'common/html_header.php';
require DIR_FS_INSTALL . DIR_WS_INSTALL_TEMPLATE . 'common/main_template_vars.php';
require DIR_FS_INSTALL . DIR_WS_INSTALL_TEMPLATE . 'common/tpl_main_page.php';
}