What is the equiv. of loading system libraries?

I tried the following:

<?php
namespace PHPMaker2025\KOMODO;

use DI\ContainerBuilder;
use Psr\Http\Message\ServerRequestInterface as Request;
use Psr\Http\Message\ResponseInterface as Response;
use Psr\Container\ContainerInterface;
use Psr\Cache\CacheItemPoolInterface;
use Doctrine\DBAL\ParameterType;
use Doctrine\DBAL\Connection;
use Doctrine\DBAL\Result;
use Doctrine\DBAL\Query\QueryBuilder;
use Doctrine\DBAL\Cache\QueryCacheProfile;
use Symfony\Component\Security\Core\User\UserInterface;
use Symfony\Component\Validator\Constraints as Assert;
use Symfony\Component\Validator\Validator\ValidatorInterface;
use Dflydev\FigCookies\FigRequestCookies;
use Dflydev\FigCookies\FigResponseCookies;
use Dflydev\FigCookies\SetCookie;
use Slim\Interfaces\RouteCollectorProxyInterface;
use Slim\App;
use League\Flysystem\DirectoryListing;
use League\Flysystem\FilesystemException;
use Closure;
use DateTime;
use DateTimeImmutable;
use DateInterval;
use Exception;
use InvalidArgumentException;

// Autoload
require_once "../vendor/autoload.php";

// Require files
require_once "../src/constants.php";
require_once "../src/config.php";
require_once "../src/phpfn.php";
require_once "../src/userfn.php";

$conn = Conn();
                   
?>

what I want is DBAL access to custom file

The following error was returned:

Fatal error : Uncaught TypeError: PHPMaker2025\KOMODO\Conn(): Return value must be of type Doctrine\DBAL\Connection, null returned in /var/www/komodocodes.com/public_html/src/phpfn.php:1974 Stack trace: #0 /var/www/komodocodes.com/public_html/shop/shop_inner.php(42): PHPMaker2025\KOMODO\Conn() #1 /var/www/komodocodes.com/public_html/shop/index.php(5): include(‘…’) #2 {main} thrown in /var/www/komodocodes.com/public_html/src/phpfn.php on line 1974