It relies on a single Symfony component which has become the standard for almost every HTTP package.

You don't need to download the entire Symfony framework

Right, but that's a large, core Symfony component, not a small library. It essentially dictates how you handle fundamental tasks (such as routing) and immediately starts you off doing things the "Symfony way".

Eh, it doesn't dictate how you handle routing. I think you're misunderstanding the dependency here: what StackPHP depends on is the HttpKernel interface. Basically the function signature for a HTTP request handler.

It does tie you into the Symfony Request object, but otherwise there are no other dependencies.