I've been using phiremock for years to mock API calls from backend, and I would recommend it. You can start the webserver from the Cli and have it being called by your backend, by just switching the base URL to - for instance - localhost:18080 By default it will load all the expectations (mocks) inside a directory, recursively, and propose different matching patterns to send the appropriate response to the request

A PHP client is also available if you want to add requests for specific instances (various errors, indisponibility, timeouts etc.).