On of the standards that have come out of the PHP-FIG (PHP Framework Interoperability Group) in the past few years has been PSR-7, a standards definition for working with HTTP requests and responses as PHP objects. While those that have worked with most of the PHP frameworks out there may be familiar with the concept, it can be confusing if you're just getting started with the idea. In this post on the Dotkernel site they introduce PSR-7, talking about its goals and what it defines to help bring everyone on the same page for HTTP requests.

PSR-7 is a set of common interfaces defined by PHP Framework Interop Group. These interfaces are representing HTTP messages, and URIs for use when communicating trough HTTP.

Any web application using this set of interfaces is a PSR-7 application.

They start off by defining (and linking to) the different interfaces involved in the PSR-7 specification (the spec doesn't define functionality, only the structure). From there the tutorial uses the Zend Diactoros component to illustrate an implementation of the PSR-7 structure. They cover two of the main tasks when working with HTTP requests/responses: working with the headers and fetching/writing to the body.