ThinPHP is a lightweight, flexible PHP5 MVC framework. It aims to be a fast, simple and highly extensible framework. It is designed to allow RAD (Rapid Application Development), help you jump start your project, start coding your ideas in minutes with minimal learning curve.
ThinPHP is small (less than 50KB zipped), has fewer classes than other frameworks and library independent (excluded core view lib - Smarty). It incorporates a lot of useful features and best practices to help you in the process of web development.
Source code is available to download, it's easy to read and extend. No more learning curve and it means you get more time to implement your project.