Как добавить иконку в zend form?

$this->add([
            'name' => 'submit',
            'type' => 'Submit',
            'attributes' => [
                'value' => 'Search',
                'id' => 'submit',
                'class' => 'btn btn-md btn-block btn-primary btn-daily-subscribed-users'
            ]
        ]);