Difference between #2 and #3 of Enforcing "You must agree" requirements in forms

Checkboxes have two possible values, `true` and `false`, but some applications
require that the user select one particular value before proceeding. Examples
might be:

* I agree to the terms and conditions
* I agree not to ship these goods into a terrorist country
* I agree that Yii is the best MVC framework ever
* I am over 18 years of age

In this case, the checkbox (implemented in the usual way in a form) warrants a
validator that enforces this condition, and it can be done with the built-in
`compare` validator along with a custom message:

Here, the model's `iagree` attribute (which could be named anything as your
database or form dictate) can be attached to a form, and validation will
be applied to it:
~~~
[php]
class SomeModel extends CActiveRecord {
    ...
    public function rules()
    {
        array('iagree', 'compare', 'compareValue' => true, 
              'message' => 'You must agree to the terms and conditions' ),
        ...
    }
    ...
}
~~~
Be sure to change the `message` to more precisely reflect what the user is
agreeing to, as well as to provide an `error` label in the form.