$fieldset = $form->addFieldset('fieldset_name', array('legend' => 'Fieldset Legend'));
$fieldset = $form->addFieldset('fieldset_name', array( 'legend' => 'Fieldset Legend', 'class' => 'fieldset_class', 'collapsable' => true) );This example adds a fieldset with a name 'fieldset_name', a legend 'Fieldset Legend', a class 'fieldset_class', and a collapsible option set to true. Both examples use the Varien_Data_Form class and are part of the Magento package library.