(Volley) How to receive Map<String,String> parameters with PHP?

public class GsonGetRequest_Exp2<T> extends Request<T>
{

    private final Gson gson;
    private final Type type;
    private Map<String, String> params;
    private final Response.Listener listener;

    public GsonGetRequest_Exp2
            (String url, Type type, Map<String,String> params, Gson gson,
             Response.Listener<T> listener, Response.ErrorListener errorListener)
    {
        super(Method.GET, url, errorListener);

        this.params     = params;
        this.gson       = gson;
        this.type       = type;
        this.listener   = listener;
    }


    @Override
    public Map<String, String> getParams() {
        return params;
    }



    @Override
    protected void deliverResponse(T response)
    {
        listener.onResponse(response);
    }

    @Override
    protected Response<T> parseNetworkResponse(NetworkResponse response)
    {
        try
        {

            String json = new String(response.data, HttpHeaderParser.parseCharset(response.headers));

            return (Response<T>) Response.success
                    (
                            gson.fromJson(json, type),
                            HttpHeaderParser.parseCacheHeaders(response)
                    );
        }
        catch (UnsupportedEncodingException e)
        {
            return Response.error(new ParseError(e));
        }
        catch (JsonSyntaxException e)
        {
            return Response.error(new ParseError(e));
        }
    }

}