Bittrex API request

I'm trying to get my wallet balance with the bittrex API and i don't understand why the code provided by bittrex documentation doesn't work :

$apikey = 'xxx';
$apisecret = 'xxx';
$nonce = time();
$uri = 'http://ift.tt/1z78eXQ' .
    $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);

The curl_exec function returns false, i don't understand why. Thank for your help !