# PSR 7 and Value Objects Slim supports [PSR-7](https://github.com/php-fig/http-message) interfaces for its Request and Response objects. This makes Slim flexible because it can use *any* PSR-7 implementation. For example, a Slim application route does not *have* to return an instance of `\Slim\Http\Response`. It could, for example, return an instance of `\GuzzleHttp\Psr7\CachingStream` or any instance returned by the`\GuzzleHttp\Psr7\stream_for()` function. Slim provides its own PSR-7 implementation so that it works out of the box. However, you are free to replace Slim’s default PSR 7 objects with a third-party implementation. Just override the application container’s `request` and `response` services so they return an instance of`\Psr\Http\Message\ServerRequestInterface` and`\Psr\Http\Message\ResponseInterface`, respectively. ## Value objects Slim’s Request and Response objects are [*immutable value objects*](http://en.wikipedia.org/wiki/Value_object). They can be “changed” only by requesting a cloned version that has updated property values. Value objects have a nominal overhead because they must be cloned when their properties are updated. This overhead does not affect performance in any meaningful way. You can request a copy of a value object by invoking any of its PSR 7 interface methods (these methods typically have a `with` prefix). For example, a PSR 7 Response object has a`withHeader($name, $value)` method that returns a cloned value object with the new HTTP header. ~~~ <?php $app = new \Slim\App; $app->get('/foo', function ($req, $res, $args) { return $res->withHeader( 'Content-Type', 'application/json' ); }); $app->run(); ~~~ The PSR 7 interface provides these methods to transform Request and Response objects: * `withProtocolVersion($version)` * `withHeader($name, $value)` * `withAddedHeader($name, $value)` * `withoutHeader($name)` * `withBody(StreamInterface $body)` The PSR 7 interface provides these methods to transform Request objects: * `withMethod($method)` * `withUri(UriInterface $uri, $preserveHost = false)` * `withCookieParams(array $cookies)` * `withQueryParams(array $query)` * `withUploadedFiles(array $uploadedFiles)` * `withParsedBody($data)` * `withAttribute($name, $value)` * `withoutAttribute($name)` The PSR 7 interface provides these methods to transform Response objects: * `withStatus($code, $reasonPhrase = '')` Refer to the [PSR-7 documentation](http://www.php-fig.org/psr/psr-7/) for more information about these methods.