I really think you have a wrong definition (or imagination) on this topic and this has nothing to do with Yii.
It is simple: You can not. This is the purpose of 1-way mathematical(!) functions that provide a one-way street to hash passwords (and other things). You can not revert this process (unless you are a hacker with spare time and rainbow tables )
I recommend you do your homework carefully and when and if you decide on hashing your passwords, as the first answer in the mentioned stackoverflow suggests, I recommend going with phpass extension. Be sure to read the comments of that extension as well (in the link given).