This demo shows how to implement an api client based on PSR-7 (RequestInterface/ResponseInterface), PSR-17 (RequestFactoryInterface, ResponseFactoryInterface) and PSR-18 (ClientInterface).
When relying solely on PSR's, the user is free to choose whatever http client he/she wants. For example, symfony/http, guzzlehttp/guzzle or similar. The only requirement is that those http clients support the PSR's either natively or via adapters.