Symfony: how to debug unexpected '\' (T_NS_SEPARATOR) - symfony

Question

I am getting the following error message in Symfony:
Parse error: syntax error, unexpected '\' (T_NS_SEPARATOR), expecting identifier (T_STRING)
when I execute DoctrineMigrations for example this one line shows up?
how can I debug this?
this is the full trace:
PHP Fatal error: Uncaught Symfony\Component\Debug\Exception\FatalThrowableError: Parse error: syntax error, unexpected '\' (T_NS_SEPARATOR), expecting identifier (T_STRING) in /Library/WebServer/Documents/hrd_3/var/cache/dev/appDevDebugProjectContainer.php:7087
Stack trace:
#0 /Library/WebServer/Documents/hrd_3/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/Kernel.php(117): Symfony\Component\HttpKernel\Kernel->initializeContainer()
#1 /Library/WebServer/Documents/hrd_3/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Console/Application.php(68): Symfony\Component\HttpKernel\Kernel->boot()
#2 /Library/WebServer/Documents/hrd_3/vendor/symfony/symfony/src/Symfony/Component/Console/Application.php(117): Symfony\Bundle\FrameworkBundle\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#3 /Library/WebServer/Documents/hrd_3/bin/console(29): Symfony\Component\Console\Application->run(Object(Symfony\Component\Console\Input\ArgvInput))
in /Library/WebServer/Documents/hrd_3/var/cache/dev/appDevDebugProjectContainer.php on line 7087
Fatal error: Uncaught Symfony\Component\Debug\Exception\FatalThrowableError: Parse error: syntax error, unexpected '\' (T_NS_SEPARATOR), expecting identifier (T_STRING) in /Library/WebServer/Documents/hrd_3/var/cache/dev/appDevDebugProjectContainer.php:7087
Stack trace:
#0 /Library/WebServer/Documents/hrd_3/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/Kernel.php(117): Symfony\Component\HttpKernel\Kernel->initializeContainer()
#1 /Library/WebServer/Documents/hrd_3/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Console/Application.php(68): Symfony\Component\HttpKernel\Kernel->boot()
#2 /Library/WebServer/Documents/hrd_3/vendor/symfony/symfony/src/Symfony/Component/Console/Application.php(117): Symfony\Bundle\FrameworkBundle\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#3 /Library/WebServer/Documents/hrd_3/bin/console(29): Symfony\Component\Console\Application->run(Object(Symfony\Component\Console\Input\ArgvInput))
in /Library/WebServer/Documents/hrd_3/var/cache/dev/appDevDebugProjectContainer.php on line 7087
update:
in the debug container on 7087:
class VichUploaderBundleHandlerDownloadHandler_00000000110518fa000000015e248fd4966f7bb604f68fe14d5ab23e4a657cd2 extends \\Vich\UploaderBundle\Handler\DownloadHandler implements \\ProxyManager\Proxy\VirtualProxyInterface
{
if i disable this module, then another class will have this problem... like randomly.

Answer 1

I've found that the issue is caused by new zendframework v3.0.3 - see https://github.com/symfony/symfony/issues/9815
So for the quick fix at the moment just add
"zendframework/zend-code": "3.0.2"
into your composer file

Answer 2

Same bug is in symfony/framework-bundle version 2.8.13, file DependencyInjection/FrameworkExtension.php,
line 374:
$container->setParameter('router.cache_class_prefix', $container->getParameter('kernel.container_class'));
Version 2.8.12 is OK:
$container->setParameter('router.cache_class_prefix', $container->getParameter('kernel.name').ucfirst($container->getParameter('kernel.environment')));
Tested on PHP 5.6.26 ...