Symfony 2.0 to 2.3 update, buildForm

...FormType::buildForm() must be compatible with Symfony\Component\Form\FormTypeInterface::buildForm(Symfony\Component\Form\FormBuilderInterface $builder, array $options) in ...

change:

use Symfony\Component\Form\FormBuilder;
...
public function buildForm(FormBuilder $builder, array $options)

to:

use Symfony\Component\Form\FormBuilderInterface;
...
public function buildForm(FormBuilderInterface $builder, array $options)