HMAC Conversion from PHP to C#

Hi,

I have a script in PHP which generates a signed string for an API, and I need to convert it to C#.  The output is not matching somehow though, and so the API is returning a 401.  What am I missing?

PHP:
signature = hash_hmac('sha256', 'GET' . _url . hash('sha256', "") . timestamp . nonce, $secret_key);

C#
var signature = SignWithHMAC("GET" + _url + CreateHash("") + timestamp + nonce, secret_key);

 public static string SignWithHMAC(byte[] dataToSign, byte[] keyBody)
        {
            var enc = Encoding.ASCII;
            HMACSHA256 hmac = new HMACSHA256(keyBody);
            hmac.Initialize();

            byte[] buffer = dataToSign;
            return Convert.ToBase64String(hmac.ComputeHash(buffer)).Replace("-", "");
        }

 public static byte[] CreateHash(byte[] data)
        {
            SHA256 shaM = new SHA256Managed();
            byte[] result = shaM.ComputeHash(data);
            return result;
        }

Is there any reason the two different hashing and hmac methods would return different results?

Thanks