| MessageInterface | HTTP messages consist of requests from a client to a server and responses from a server to a client. This interface defines the methods common to each. |
| RequestInterface | Representation of an outgoing, client-side request. |
| ResponseInterface | Representation of an outgoing, server-side response. |
| ServerRequestInterface | Representation of an incoming, server-side HTTP request. |
| StreamInterface | Describes a data stream. |
| UploadedFileInterface | Value object representing a file uploaded through an HTTP request. |
| UriInterface | Value object representing a URI. |