为什么activitieDropdownlist的值始终无法获取到？ [ 新手入门 ]

view层

<form method="post">
    <?php echo CHtml::activeDropDownList($m_subjectlist,'subject_name',$sb_list);?>
<div class="row submit">
    <?php echo CHtml::submitButton('显示我所选择的专业'); ?>
</div>
 <?php  echo 'test'.$result.'111';?>
</form>

controller控制器层

$result=$_POST['subject_name'];
$this->render('dbdplist',array(  
    'result'=>$result,
    'model'=>$model,));