Slim is a miniature PHP framework that weighs a mere 38 kilobytes.
Slim supports RESTful routing, and associates each one of these routes with HTTP requests (GET, POST, etc), and has additional features such as extensive error handling.
Slim has quite a few solid core features for its small size.
The framework eases coding significantly, but does miss out on the functionality possessed by more popular frameworks.
For other small PHP frameworks, check out our awesome post on them!