This document discusses dependency inversion in PHP using composer and virtual packages. It describes how a developer can create an API client that depends on abstractions rather than specific implementations by using virtual packages. This allows the client code to remain decoupled from the underlying HTTP client implementation and avoid version conflicts. The document provides examples of how to declare a virtual package in composer.json that implements interfaces from packages like HTTP-Plug, and how clients can then require the virtual package to gain the benefits of abstraction and flexibility in choosing implementations.