Its very common now-a-days to have Mobile app for web apps. For Mobile apps we need web APIs to fetch data or even post/update on web. We found a very simple method to make such interface.
Mobile App can call plain get or post request and receive data back in JSON format. JSON is relatively easy for mobile app to handle while plain standard GET/POST request for easy for Web server to handle. This mixed approach saves lot of time on server side. We not really need to handle actual REST requests and still be able to exchange data between web and mobiles or ajax java-scripts or even node.js.
To extend the functionality of you web application relative to your requirements you are supposed to use existing yii core libraries or use external libraries. There are some steps to ensure security, uniqueness, modularity, performance and to avoid rework in future.