mkdir my-slim-api && cd my-slim-api
composer init

{
  "name": "zhukmax/slim",
  "type": "project",
  "license": "MIT",
  "autoload": {
    "psr-4": {
      "Zhukmax\\Slim\": "src/"
    }
  },
  "require": {}
}

composer require slim/slim:"4.*"
composer require slim/psr7

<?php
require __DIR__ . '/vendor/autoload.php';
use Psr\Http\Message\ResponseInterface as Response;
use Psr\Http\Message\ServerRequestInterface as Request;
use Slim\Factory\AppFactory;
$app = AppFactory::create();
$app->get('/', function (Request $request, Response $response, $args) {
  $data = array('name' => 'Max', 'role' => 'web developer');
  $response->getBody()->write(json_encode($data));
  return $response->withHeader('Content-Type', 'application/json');
});
$app->run();

"require": {
  "slim/slim": "4.*",
  "slim/psr7": "^1.4",
  "ext-json": "*"
}

composer require j4mie/idiorm

<?php
ORM::configure([
  // Обратите внимание, что localhost не работает по умолчанию,
  // прежде чем паниковать попробуйте 127.0.0.1
  'connection_string' => 'mysql:host=127.0.0.1;dbname=mydb',
  'username' => 'root',
  'password' => '124'
]);
$app->get('/users', function (Request $request, Response $response, $args) {
  $users = ORM::forTable("users")->find_array();
  $response->getBody()->write(json_encode($users));
  return $response;
});

<?php
$app->post('/users', function (Request $request, Response $response, $args) {
  $parsedBody = $request->getParsedBody();
  $user = ORM::forTable("users")->create();
  $user->name = $parsedBody['name'] ?? '';
  if ($user->save()) {
    $successRes = $response->withStatus(201);
    $successRes->getBody()->write(json_encode([
      "message" => "Success"
    ]));
    return $successRes;
  } else {
    $errorRes = $response->withStatus(501);
    $errorRes->getBody()->write(json_encode([
      "message" => "Error"
    ]));
    return $errorRes;
  }
});

$app->get('/users/{id}', [UserController::class, "getOne"]);

<?php
class UserController
{
  public function getOne(Request $request, Response $response, $args): Response
  {
    $id = (int)$args['id'] ?? 0;
    $user = ORM::forTable("table1")->findOne($id);
    if (!$user) {
      return self::errorResponse($response, 404, "Error text");
    }
    $response->getBody()->write(json_encode([
      "id" => $user->id,
      "name" => $user->name
    ]));
    return $response;
  }
  private static function errorResponse(Response $response, int $code, string $text): Response
  {
    $errorRes = $response->withStatus($code);
    $errorRes->getBody()->write(json_encode([
      "message" => $text
    ]));
    return $errorRes;
  }
}