Building extensible PHP apps with Symfony DI

In building complex PHP applications, dependency injection and service containers play a crucial role in managing object instantiation, with the Symfony's DependencyInjection library standing out due to its power, simplicity, and extensive documentation. This library allows for the automation of service initialization through compiler passes, which modify service behavior before container compilation. It enhances extensibility by enabling third-party developers to inject their services without needing a deep understanding of the application's inner workings, thus reducing documentation needs. The process involves automatically initializing services that implement specific interfaces, thereby streamlining efforts for developers. Additionally, services can be registered differently depending on the application's context, such as admin or user-facing sides, and conditional service initialization can be determined at runtime. This approach aligns with the principle of convention over configuration, significantly simplifying the management of complex applications and extensions by reducing configuration requirements and leveraging logical folder structures to convey service availability and initialization contexts.