Payum is a framework-agnostic PHP payment processing library that unifies dozens of payment providers (PayPal, Stripe, Klarna, Authorize.Net, Be2Bill, Payex, Sofort, Offline, and others) behind a common API. The code reveals a dual architecture in transition: the mature 1.x action/request/reply model coexists with a newer 2.x command/handler/context model that returns typed result objects and executes through a middleware pipeline. Core capabilities include gateway factories, registry and container abstractions, token-based security URLs, pluggable storage backends (Doctrine ORM, MongoDB ODM, filesystem, Laminas), Twig template rendering, PSR-7/14/18 support, event dispatching, and currency/money handling. It is a library meant to be embedded inside Symfony, Laravel, or plain PHP applications, not a standalone service. The architecture is genuinely well-engineered: the middleware pipeline, handler map, token factory, and storage abstractions are cleanly separated and heavily unit-tested. However, the codebase is mid-migration between 1.x and 2.x APIs, with many deprecated legacy adapters and bridge classes still present. This dual-path design increases complexity and can confuse new users, but the core abstractions remain sound and reusable. The repository is a monorepo with per-gateway subpackages, extensive PHPUnit coverage, CI workflows, static analysis, and release automation. Technical drift is moderate: gateway integrations using old SOAP/XML-RPC SDKs and deprecated Symfony/Httplug bridges are aging, while the core 2.x architecture is actively maintained.