货币转换器API显示空值

Question

我目前在需要货币转换器的网站上工作，我为货币转换器尝试了很少的php APi，但没有一个能获得空值。

    function currencyConverter($currency_from, $currency_to, $currency_input) {
        $yql_base_url = "http://query.yahooapis.com/v1/public/yql";
        $yql_query = 'select * from yahoo.finance.xchange where pair in ("' . $currency_from . $currency_to . '")';
        $yql_query_url = $yql_base_url . "?q=" . urlencode($yql_query);
        $yql_query_url .= "&format=json&env=store%3A%2F%2Fdatatables.org%2Falltableswithkeys";
        $yql_session = curl_init($yql_query_url);
        curl_setopt($yql_session, CURLOPT_RETURNTRANSFER, true);
        $yqlexec = curl_exec($yql_session);
        $yql_json =  json_decode($yqlexec, true);
        $currency_output = (float) $currency_input * $yql_json['query']['results']['rate']['Rate'];

        return $currency_output;
    }

    $currency_input = 10;
    $currency_from = "USD";
    $currency_to = "INR";
    $currency = currencyConverter($currency_from, $currency_to, $currency_input);
    echo $currency_input . '' . $currency_from . ' = ' . $currency . ' ' . $currency_to;
?>