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 can be attached to a form, and validation will be applied to it:

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.