With:

<?php

class StringCodeException extends Exception
{
public function __construct()
{
$this->code = 'SomeString';
}
}

$e = new StringCodeException;

try {
throw $e;
} catch (StringCodeException $e) {
var_dump($e->getCode());
}
?>

With xdebug loaded:
int(0)

otherwise:
string(10) "SomeString"

(As per https://github.com/sebastianbergmann/phpunit/issues/1972#issuecomment-160537077)