Go\Instrument\ClassLoading\SourceTransformingLoader::getId PHP Method

    public static function getId()
    {
        if (empty(self::$filterId)) {
            throw new \RuntimeException('Stream filter was not registered');
        }
        return self::$filterId;
    }

 protected function registerTransformers()
 {
     $sourceTransformers = array(new FilterInjectorTransformer($this->options, SourceTransformingLoader::getId()), new BeforeMockTransformer($this, new TokenReflection\Broker(new TokenReflection\Broker\Backend\Memory()), $this->container->get('aspect.advice_matcher')));
     return array(new CachingTransformer($this, $sourceTransformers));
 }