Atualização do Symfony 2.0 para 2.3, buildForm

...FormType::buildForm() must be compatible with SymfonyComponentFormFormTypeInterface::buildForm(SymfonyComponentFormFormBuilderInterface $builder, array $options) in ...

mudança:

use SymfonyComponentFormFormBuilder;
...
public function buildForm(FormBuilder $builder, array $options)

para:

use SymfonyComponentFormFormBuilderInterface;
...
public function buildForm(FormBuilderInterface $builder, array $options)