The Inviqa blog has posted a tutorial that gets in to the details of using PSR-7 compatible functionality in Symfony through the introduction of middleware into your application.

The PSR-7 standard, which describes common HTTP message interfaces, is a big step towards interoperability across different PHP libraries. The standard was introduced not long ago, but you can already use libraries compatible with this recommendation within your Symfony-based application.

[...] A step toward more homogeneity was achieved when the PHP Framework Interop Group accepted PSR-7 in May 2015. This recommendation describes common HTTP message interfaces. The biggest benefit the PHP community gets from the standard is a potential for interoperability across different PHP libraries. You can already use libraries compatible with this recommendation within your Symfony-based application thanks to the Symfony PSR-7 Http Message Bridge.

The tutorial then shows how to use this message bridge to convert the current Symfony HTTP request and response instances over to follow the PSR-7 structure (essentially a wrapper around it). They then show how to use this functionality in a simple Symfony controller, taking advantage of an event listener to automatically convert the request based on type hinting in the controller method. Finally they talk about middleware, what they are and how they fit into the flow of a web request/response structure.