How can I change the attributes of a input form? 

I create with this a input (productform.php):

```
$this->add(array(
                'name' => 'categoryId',
                'attributes' => array(
                        'id' => 'categoryId',
                        'type' => 'hidden',
                        'value' => '',
                ),
        ));

```

In a previous page I link to the form and set the special value in the url (`....com/form/3`). 

In the indexcontroller.php I get the form with `$form = new ProductForm();` and want edit the value and set the special value from the url. 

My idea was the `$form->setAttribute('categoryId', 'value');` but that not working.

Thanks.

indexcontroller.php

```
...
        $form = new ProductForm();
        $form->setHydrator(new CategoryHydrator());
        $form->bind(new Product());
        $form->setAttribute('categoryId', 'value');
....

```

productform.php

```
...

class ProductForm extends Form
{
    public function __construct()
    {
        parent::__construct('productForm');
        $this->setAttribute('action', 'newproduct');
        $this->setAttribute('method', 'post');

        $this->add(array(

........

```