PHP developers love Slim for developing Restful APIs and services, thanks to Josh Lockhart for creating this tiny beast. The best thing about starting with a micro framework like slim is that you don’t need to spend time learning it. The code base is so small that you can run through the entire code in a late night session.
Slim is an open source, sophisticated micro framework that comes with URL routing and best in class middleware architecture. It also supports encrypted cookies, flash messages, page templates and caching apart from many other features.
Slim framework is best suited in scenarios where you want to build simple applications or API’s grounds up with full flexibility in your hands. Slim resides at the core of your application and you write the code around as required or plug existing components from other frameworks to create fully functional web application.
Silex and Lumen are two other frameworks similar to Slim that fall into category of micro PHP frameworks.
You can read more about Slim at - Slimframework.com.