PSR15

PSR-15, the PHP Standard Recommendation for HTTP Server Request Handlers, is a specification that standardizes server-side HTTP middleware and request handlers in PHP applications. It defines interfaces that allow middleware components to be composed into a request-handling pipeline and to interoperate across frameworks and libraries.

The core interfaces are Psr\Http\Server\MiddlewareInterface and Psr\Http\Server\RequestHandlerInterface. MiddlewareInterface defines a single method, process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface. RequestHandlerInterface defines handle(ServerRequestInterface $request): ResponseInterface. Both interfaces rely on PSR-7 message interfaces for requests and responses, namely Psr\Http\Message\ServerRequestInterface and ResponseInterface.

Usage within a pipeline is straightforward: a middleware component implements MiddlewareInterface and can either return a ResponseInterface to short-circuit the pipeline or delegate to the next component by calling $handler->handle($request). The final outcome is a ResponseInterface produced by either the last middleware or the final request handler.

Impact and ecosystem: PSR-15 provides a common contract that enables middleware reuse across frameworks. It is widely supported by PHP frameworks and libraries, and many projects offer PSR-15 compatible middleware or adapters to integrate with different stacks.

Relation to other standards: PSR-15 complements PSR-7 for HTTP message objects and PSR-17 for factories. It does not define how requests are received by servers, but standardizes how middleware and handlers interact within a server-side pipeline.