Developing web applications you often need to have multiple configuration files for different development environments (DEV, QA, UAT, PROD etc). This can be solved by changing config file on the developers’ local machine, but this method has its drawbacks. For example, you should always ensure that while using VCS you will not accidentally commit your changes and which is not very comfortable from team development perspective.
Full article here - http://weavora.com/blog/2011/11/22/yii-framework-custom-environment/#more-322