CakePHP 2.4.4如果选中复选框，则显示并验证另一个字段

Question

My question is similar to: cakePHP - form - if checkbox is selected show and validate another field.
I have a set of fields in a form:

/* view */
echo $this->Form->input('immagine', array('type' => 'checkbox'));
echo $this->Form->input('url');

and the model, with the validation rules:

/* model */
class Question extends AppModel {
    public $validate = array(
            'url' => array(
                'required_if_checked'=>array(
                    'rule'=>array('requiredIfChecked', 'immagine'),
                    'message'=>'cannot be blank if foo is checked',
                )
            )
        );
    public function requiredIfChecked($check, $associatedField) {
        $value = array_pop($check);
        if ($this->data[$this->alias][$associatedField] && empty($value)) {
            return false;
        }
        else {
            return true;
        }
    }
}

On the page, the url field is marked as required either the checkbox is ticked or not.
I've tried adding

'required' => false,
'allowEmpty' => true

in the model, but that just does what it says, ignoring the validation rules.

I want the 'url' field to be validated ONLY if the 'immagine' checkbox is ticked.

Thank you all for help.