Does Amazon S3 have a limit to MaxKeys when calling ListObjects?

The S3 API limit hasn't changed, it's still limited to a maximum of 1000 keys/response.

With the PHP SDK v1 a single request returned up to 1000 keys and to get the rest you needed to do a second request with the marker option.

The new PHP SDK (v2) has a concept of Iterators which abstracts the process of doing these multiple, consecutive requests. This makes getting ALL of your objects much easier.