symfony/http-kernel
Http Kernel
HttpKernel is the core kernel component of the Symfony framework that orchestrates the complete HTTP request-response lifecycle. It provides an event-driven pipeline that converts an incoming HTTP request into a matching response by dispatching lifecycle events for early interception, controller resolution, error handling, response modification, and deferred tasks.
The kernel automatically resolves the PHP callable that handles a request and injects its arguments using reflection and request attributes. It dispatches attribute-specific events for each PHP attribute found on a controller, enabling targeted listener responses without manual inspection. When exceptions occur during the request cycle, the kernel catches them and dispatches an event for listeners to return custom error responses. It also supports converting non-Response controller return values into proper Response objects through a view event, and allows modification of response headers, cookies, or body content just before sending.
A distinctive capability is the sub-request nesting mechanism, which allows a full request-response cycle to be executed inside another request for rendering embedded page fragments like widgets. The kernel also triggers a deferred termination event after the response has been sent to the client, enabling heavy tasks such as email delivery without blocking the request. After each request cycle, the kernel resets accumulated state in services to prevent memory leaks in long-running processes.
