In the API I'm currently building, I'm rendering errors using RFC 7807: Problem Details for HTTP APIs. As this is a Slim Framework project, it uses PSR-7, so I updated rka-content-type-renderer to support problem.
RFC 7807 defines a standard for sending details of an error in an HTTP response message. It supports both XML and JSON formats.
He starts with an example of the "Problem" response format that includes data for the type of error, details and links to other related objects. He points out this package from Larry Garfield that handles the actual output of the respose format but Rob needed a way to shift between JSON and XML formats too. This is where his updates to his package came in, changing it to include a
ApiProblemRenderer that reads the "Accept" header of the incoming request and correctly formats the results accordingly.