Symfony\Component\Form\AbstractType::setDefaultOptions PHP Method

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults($this->getDefaultOptions(array()));
        $resolver->addAllowedValues($this->getAllowedOptionValues(array()));
    }

 /**
  * @param \Symfony\Component\OptionsResolver\OptionsResolverInterface $resolver
  *
  * @return void
  */
 public function setDefaultOptions(OptionsResolverInterface $resolver)
 {
     parent::setDefaultOptions($resolver);
     $resolver->setRequired('action');
     $resolver->setRequired('fields');
     $resolver->setDefaults(['attr' => ['class' => 'form-inline']]);
 }