Barebone example of a PSR-15 middleware stack.

composer.json

{
    "require": {
        "psr/http-server-middleware": "^1.0",
        "zendframework/zend-diactoros": "^1.7"
    }
}

index.php

<?php

declare(strict_types=1);

use Psr\Http\Message\ResponseInterface as Response;
use Psr\Http\Message\ServerRequestInterface as Request;
use Psr\Http\Server\MiddlewareInterface as Middleware;
use Psr\Http\Server\RequestHandlerInterface as Handler;
use Zend\Diactoros\Response\SapiEmitter;
use Zend\Diactoros\Response\TextResponse;
use Zend\Diactoros\ServerRequestFactory;

require 'vendor/autoload.php';

(function(){
    $request = ServerRequestFactory::fromGlobals();

    $app = new class implements Handler
    {
        public function handle(Request $request): Response
        {
            $greet = $request->getQueryParams()['greet'] ?? "World";

            return new TextResponse("Hello $greet");
        }
    };

    $middlewareStack = [
        new class implements Middleware {
            public function process(Request $request, Handler $handler): Response
            {
                if (! isset($request->getQueryParams()['greet'])) {
                    throw new Exception('missing greet query param!');
                }
                return $handler->handle($request);
            }
        },
        new class implements Middleware {
            public function process(Request $request, Handler $handler): Response
            {
                try {
                    $response = $handler->handle($request);
                } catch (Exception $exception) {
                    $response = new TextResponse('Error: ' . $exception->getMessage(), 500);
                }

                return $response;
            }
        },
    ];

    $middlewarePipe = array_reduce(
        $middlewareStack,
        function(Handler $next, Middleware $middleware)
        {
            return new class($next, $middleware) implements Handler
            {
                private $next;
                private $middleware;
                public function __construct(Handler $next, Middleware $middleware)
                {
                    $this->next = $next;
                    $this->middleware = $middleware;
                }
                public function handle(Request $request): Response
                {
                    return $this->middleware->process($request, $this->next);
                }
            };
        },
        $app
    );

    $response = $middlewarePipe->handle($request);

    (new SapiEmitter())->emit($response);
})();