www.yiiframework.com/doc/api/1.1/CHtml#activeCheck...
A special option named 'uncheckValue' is available that can be used to specify the value returned when the checkbox is not checked. By default, this value is '0'. Internally, a hidden field is rendered so that when the checkbox is not checked, we can still obtain the posted uncheck value.
If 'uncheckValue' is set as NULL, the hidden field will not be rendered.
