You may also use the \Slim\Middleware\SessionCookie middleware to persist session data in encrypted, hashed HTTP cookies. To enable the session cookie middleware, add the \Slim\Middleware\SessionCookie middleware to your Slim application:

<?php
$app = new Slim();
$app->add(new \Slim\Middleware\SessionCookie(array(
    'expires' => '20 minutes',
    'path' => '/',
    'domain' => null,
    'secure' => false,
    'httponly' => false,
    'name' => 'slim_session',
    'secret' => 'CHANGE_ME',
    'cipher' => MCRYPT_RIJNDAEL_256,
    'cipher_mode' => MCRYPT_MODE_CBC
)));

The second argument is optional; it is shown here so you can see the default middleware settings. The session cookie middleware will work seamlessly with the $_SESSION superglobal so you can easily migrate to this session storage middleware with zero changes to your application code.

If you use the session cookie middleware, you DO NOT need to start a native PHP session. The $_SESSION superglobal will still be available, and it will be persisted into an HTTP cookie via the middleware layer rather than with PHP’s native session management.

Remember, HTTP cookies are inherently limited to only 4 kilobytes of data. If your encrypted session data will exceed this length, you should instead rely on PHP’s native sessions or an alternate session store.

PLEASE NOTE: Client-side storage of session data is not recommended if you are dealing with sensitive information, even when using Slim's encrypted session cookie middleware. If you need to store sensitive information, you should encrypt and store the session information on your server.