Description:
------------
On Debian Jessie (9) with PHP 7.0.27-0+deb9u1 (cli), it is impossible to use the character '?' in a function declaration.

PHP7.0.27 package is the most recent on Debian 9. I have not found a fix :(

Test script:
---------------
    private function addCityField(FormInterface $form, ?Department $department) {
        $form->add('city', EntityType::class, [
            'class'         => 'AppBundle\Entity\City',
            'placeholder'   => $department ? 'Select your city' : 'Select your department',
            'choices'       => $department ? $department->getCities() : []
        ]);
    }

Expected result:
----------------
Parse error: syntax error, unexpected '?', expecting variable (T_VARIABLE)

Line with problem :
"private function addCityField(FormInterface $form, ?Department $department) {"