$apikey='xxx';
$apisecret='xxx';
$nonce=time();
$uri='https://bittrex.com/api/v1.1/market/getopenorders?apikey='.$apikey.'&nonce='.$nonce;
$sign=hash_hmac('sha512',$uri,$apisecret);
$ch = curl_init($uri);
curl_setopt($ch, CURLOPT_HTTPHEADER, array('apisign:'.$sign));
$execResult = curl_exec($ch);
$obj = json_decode($execResult);