This is only a convention to make it easier to find the input elements that belong to a specific form. Check out the source code of forms created by Yii: All input elmements will have names like LoginForm[username] instead of username. So no special PHP configuration is required.
BTW, I didn’t use Yii’s Helper class to generate the form. Instead, I just used pure html code. So, no wonder how I searched with google but just cannot found the answer.
Of course you can. But then you loose some of Yii’s nice convenience features and some things might not work anymore or be harder to achieve… So - as you’re beginning with Yii - i’d advise not to do so. Is there any reason you don’t want to use it?