Exception->getCode() is allowed to return mixed.But unfortunately it seems impossible to test against something else than integer exception code because I cannot create such an exception.
This is not allowed: new \Exception('', 'foo'); and even mocking does not work. phpunit does not use the mocked method. Maybe phpunit bug or its impossible to mock exception completely.

$exception = $this->getMock('\Exception');
$exception->expects($this->once())
    ->method('getCode')
    ->will($this->returnValue('foo'));

@Tobion getCode is final so it cannot be mocked.

FYI, a case were exceptions are using string codes is PDOException

Ah thanks for the info about final. I know about PdoException. The thing is I cannot create an exception that returns string code myself. Or do you know how? The only way seems to be that I make a wrong sql query for example so that I than get a PdoException with string code. But that seems like a bad approach.

@fabpot The variable is named $exitCode in newer branches. Just FYI when you merge upstream.