MongoCollection::ensureDocumentHasMongoId PHP Method

    private function ensureDocumentHasMongoId(&$document)
    {
        $checkKeys = function ($array) {
            foreach (array_keys($array) as $key) {
                if (empty($key) && $key !== 0) {
                    throw new \MongoException('zero-length keys are not allowed, did you use $ with double quotes?');
                }
            }
        };
        if (is_array($document)) {
            if (!isset($document['_id'])) {
                $document['_id'] = new \MongoId();
            }
            $checkKeys($document);
            return $document['_id'];
        } elseif (is_object($document)) {
            $reflectionObject = new \ReflectionObject($document);
            foreach ($reflectionObject->getProperties() as $property) {
                if (!$property->isPublic()) {
                    throw new \MongoException('zero-length keys are not allowed, did you use $ with double quotes?');
                }
            }
            if (!isset($document->_id)) {
                $document->_id = new \MongoId();
            }
            $checkKeys((array) $document);
            return $document->_id;
        }
        return null;
    }