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

I added this:

if(!curl_exec($ch)){
    die('Error: "' . curl_error($ch) . '" - Code: ' . curl_errno($ch));
}