Thank you for your interest in the RadUpload for Silverlight.
Currently we had developed only an ASP upload handler. Of course our control is not limited to work only with our upload handler and you can implement your own handler too. We suppose such a handler can be implemented in PHP. However we never tested such an approach. Please be aware there are a lot of parameters you should carry about. We highly advise you to get the code from our handler and just to re-write it in PHP.
We hope this information will help you.
the Telerik team