Apply class to Symfony2 Form Label

I'm attempting to set the class on a class on a form label using php templates.

Here's my code:

<?php echo $view['form']->label($form['first_name'], 'First Name', array(
    'attr' => array('class' => 'control-label')
)) ?>

But here's my output:

<label class="required" for="form_first_name">First name</label>

I can find how to do it using twig, but not an example with PHP.