form.php

<?php

      $this->widgetSchema['food'] = new sfWidgetFormChoice(array(
        'choices'        => array('PHP', 'symfony', 'Doctrine', 'Propel', 'model'),
        'expanded' => false,
        'multiple' => true,
        'label'=> 'Pietanze presenti'
        ), array('class'=>'multiselect'));

        $this->setDefault('food',array('Doctrine','PHP'));
        //$this->setDefaults(array('food'>=array('Propel','PHP')));

?>