There are several methods and techniques that come handy in the case you need to call an external resource periodically or if you are waiting for a server push, but I present here an easy and straightforward one using HTML5's Server-Sent Events (SSE).
This is a reference to be used for Model rule validation and is compiled from the Yii documentation and code. The purpose is to have all the information gathered in one place instead of scattered. This reference is not an intro.
See The Definitive Guide to Yii, Declaring Validation Rules for a tutorial.
Trying to learn and help some other programmers to find their solutions on the Yii Forum (I think is a superb way to learn the framework), I was facing one challenge with an external library that a fellow programmer wanted to use -quite good indeed: PHPExcel. And what is PHPExcel?
A lot of people have written posts on integrating Yii and WordPress. This article combines goncin's approach with an article I read about integrating Symfony and WordPress and applies it all to Yii and WordPress.