If you build an API in PHP, try using zircote/swagger-php or equivalent. You will thank me later.

https://github.com/zircote/swagger-php

I used it last year when rescuing a project that derailed, in big part because of a lack of a verifiable contract between the UI and API teams.

I also like to automatically validate the schema on all requests and responses in automated acceptance tests. Then I don't have to write tests specifically for the input/output shape.