yii 模糊查询 [ 新手入门 ]

public function actionSearch()
{
    echo  $_POST[keywords];
    if(isset($_POST[keywords])){
        $model =  Product::model()->findAllBySql('select * from tbl_product where name like :keywords',array(':keywords'=>$_POST[keywords]));
    }else{
        $model = Product::model()->findAll();
    }
    $this->render('index',array(
        'model'=>$model,
    ));
}

keywords 是我获取表单的名称，提交之后，提示我Use of undefined constant keywords - assumed 'keywords' 表单的名称也要定义吗，这个在哪边定义呢