index.php

<?php

// Valid PHP Version?
$minPHPVersion = '7.3';
if (version_compare(PHP_VERSION, $minPHPVersion, '<')) {
    die("Your PHP version must be {$minPHPVersion} or higher to run CodeIgniter. Current version: " . PHP_VERSION);
}
unset($minPHPVersion);

//set the variable to 'installed' after installation
$app_state = "installed";

// we don't want to access the main project before installation. redirect to installation page
if ($app_state === 'pre_installation') {
    $domain = $_SERVER['HTTP_HOST'] . $_SERVER['SCRIPT_NAME'];

    $domain = preg_replace('/index.php.*/', '', $domain); //remove everything after index.php
    if (!empty($_SERVER['HTTPS'])) {
        $domain = 'https://' . $domain;
    } else {
        $domain = 'http://' . $domain;
    }

    header("Location: $domain./install/index.php");
    exit;
}

// Path to the front controller (this file)
define('FCPATH', __DIR__ . DIRECTORY_SEPARATOR);

// Ensure the current directory is pointing to the front controller's directory
chdir(FCPATH);

/*
 *---------------------------------------------------------------
 * BOOTSTRAP THE APPLICATION
 *---------------------------------------------------------------
 * This process sets up the path constants, loads and registers
 * our autoloader, along with Composer's, loads our constants
 * and fires up an environment-specific bootstrapping.
 */

// Load our paths config file
// This is the line that might need to be changed, depending on your folder structure.
require realpath(FCPATH . 'app/Config/Paths.php') ?: FCPATH . 'app/Config/Paths.php';
// ^^^ Change this if you move your application folder

$paths = new Config\Paths();

// Location of the framework bootstrap file.
require rtrim($paths->systemDirectory, '\\/ ') . DIRECTORY_SEPARATOR . 'bootstrap.php';

// Load environment settings from .env files into $_SERVER and $_ENV
require_once SYSTEMPATH . 'Config/DotEnv.php';
(new CodeIgniter\Config\DotEnv(ROOTPATH))->load();

/*
 * ---------------------------------------------------------------
 * GRAB OUR CODEIGNITER INSTANCE
 * ---------------------------------------------------------------
 *
 * The CodeIgniter class contains the core functionality to make
 * the application run, and does all of the dirty work to get
 * the pieces all working together.
 */

$app = Config\Services::codeigniter();
$app->initialize();
$context = is_cli() ? 'php-cli' : 'web';
$app->setContext($context);

/*
 *---------------------------------------------------------------
 * LAUNCH THE APPLICATION
 *---------------------------------------------------------------
 * Now that everything is setup, it's time to actually fire
 * up the engines and make this app do its thang.
 */

$app->run();