Installation and first run

Slim is installed as an ordinary Composer package into an empty folder. It has no installer and no command that creates a project: you make the folder and the first file yourself.

What you need first

• PHP 8.1 or newer — check with php -v;
• Composer — check with composer -V.

Installation

Create the project folder and install two packages:

mkdir blog
cd blog
composer require slim/slim:"4.*" slim/psr7

slim/slim is the framework itself. slim/psr7 provides the request and response objects; Slim will not start without them. After the command the blog folder contains:

The first file

Create the public folder by hand, and the file index.php inside it:

<?php
// public/index.php
use Psr\Http\Message\ResponseInterface as Response;
use Psr\Http\Message\ServerRequestInterface as Request;
use Slim\Factory\AppFactory;
 
require __DIR__ . '/../vendor/autoload.php';
 
$app = AppFactory::create();
 
$app->get('/', function (Request $request, Response $response) {
    $response->getBody()->write('Hello, Slim!');
 
    return $response;
});
 
$app->run();

What happens here:

1. require loads the Composer autoloader — after that the package classes are available.
2. AppFactory::create() creates the application object.
3. $app->get('/', ...) declares a route: run this function for a request to /.
4. The function writes text into the response body and returns the response.
5. $app->run() handles the current request and sends the response to the browser.

Later examples leave out the <?php line, but in a real PHP file it always comes first.

Running

PHP has a built-in web server for development. Start it from the blog folder:

php -S localhost:8080 -t public

Open http://localhost:8080 — the page shows "Hello, Slim!". The -t public option makes the public folder the site root. The server runs while the terminal window stays open; stop it with Ctrl+C.

A second page

Add one more route before the $app->run(); line:

// public/index.php
$app->get('/about', function (Request $request, Response $response) {
    $response->getBody()->write('About the blog');
 
    return $response;
});

No restart is needed: open http://localhost:8080/about and you will see "About the blog".

Common mistake

A handler must return the response object. If you forget return $response, you get a type error instead of a page. Printing with echo is also wrong: the text bypasses the response object. Everything that should appear on the page is written with $response->getBody()->write().