index.php

<?php

use Application\Middleware\MyAuthMiddleware;
use DI\Container;
use DI\ContainerBuilder;
use Monolog\Logger;
use Psr\Log\LoggerInterface;
use PDO;
use PDOException;
use Slim\AppFactory;
use function DI\create;
use function DI\autowire;

require __DIR__ . '/../vendor/autoload.php';

// Create Container & Definitions
$containerBuilder = new ContainerBuilder();
$containerBuilder->addDefinitions([
    LoggerInterface::class => create(Logger::class),
    MyAuthMiddleware::class => autowire(MyAuthMiddleware::class),
    PDO::class => function (): PDO {
      $dsn = 'mysql:dbname=testdb;host=127.0.0.1';
      $user = 'dbuser';
      $password = 'dbpass';

      try {
          return new PDO($dsn, $user, $password);
      } catch (PDOException $e) {
          echo 'Connection failed: ' . $e->getMessage();
      }
    },
]);

$container = $containerBuilder->build();

// Create App
AppFactory::setContainer($container);
$app = AppFactory::create();

// Add Deferred Auth Middleware
$app->add(MyAuthMiddleware::class);

MyAuthMiddleware.php

<?php

namespace Application\Middleware;

use PDO;
use Psr\Log\LoggerInterface;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\MiddlewareInterface;

class MyAuthMiddleware implements MiddlewareInterface
{
    private LoggerInterface $logger;
    
    private PDO $pdo;
    
    public function __construct(PDO $pdo, LoggerInterface $logger)
    {
      $this->pdo = $pdo;
      $this->logger = $logger;
    }
  
    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
    {
      // Your logic here
      // Use $this->pdo
      // User $this->logger
    }
}