PHPUnit - Serialization of Closure is not allowed in Symfony 2
by Steve Hoeksema, posted 18 Mar 2013.
If you get an error like this:
PHPUnit_Framework_Exception: PHP Fatal error:
Uncaught exception 'Exception' with message
'Serialization of 'Closure' is not allowed'
in vendor/symfony/src/Symfony/Component/HttpFoundation/Session.php:383
It might be because your unit test references an uninitialized variable or array index.
No, really.
(Thanks @ethos49)