在ChoiceType中使用choice_value选项（Symfony 3）

Question

I use Symfony 3.0.4. I wish replace the value to increment by my label. I referred to the official documentation unsuccessfully. I also tried choices_as_values unsuccessfully too :(

This is a piece of my buildForm() function :

->add(
    'fieldName',
    ChoiceType::class,
    array(
        'choices' => array(
            'Yes' => true,
            'No' => true,
            'Maybe' => true,
        ),
        'choice_value' => function ($choice) {
            return $choice;
        },
    )
)

And I have a return error 500 :

Notice: Array to string conversion

Do you have any idea? Thanks a lot