Dependent DropDown

Hi,
I'm new to yii.

I have to create a suite of dependet dropdownlist, but i need your help.

This is my code in the _form.php

<div class="row">
		<?php echo $form->labelEx($model,'mut_immobile_prov_sigla'); ?>
                <?php 
                    echo $form->dropDownList($model,'mut_immobile_prov_sigla', CHtml::listData(TabProvince::model()->findAll(), 'prov_sigla','prov_descr'), array('empty'=>'',
                            'ajax' => array(
                            'type'=>'POST', //request type
                            'url'=>CController::createUrl('DataMutuiController/dynamiccities'), //url to call.
                            //Style: CController::createUrl('currentController/methodToCall')
                            'update'=>'#mut_immobile_com_id', //selector to update
                            //'data'=>'js:javascript statement' 
                            //leave out the data key to pass all form values through
                            )
                        
                        )); 
                ?>
		<?php echo $form->error($model,'mut_immobile_prov_sigla'); ?>
	</div>

	<div class="row">
		<?php echo $form->labelEx($model,'mut_immobile_com_id'); ?>
                <?php echo $form->dropDownList($model,'mut_immobile_com_id', array(), array('empty'=>'')); ?>
		<?php echo $form->error($model,'mut_immobile_com_id'); ?>
	</div>

and this in the controller

public function actionDynamiccities()
        {
            $data=Location::model()->findAll('parent_id=:parent_id', 
                          array(':parent_id'=>(string) $_POST['mut_immobile_prov_sigla']));

            $data = CHtml::listData(TabComuni::model()->findAll(), 'com_id','com_descr');
            foreach($data as $value=>$name)
            {
                echo CHtml::tag('option', array('value'=>$value),CHtml::encode($name),true);
            }
        }

I have copied it from the tutorial but it won't work.

Anyone can help me please?

Thank you