Paypal PDT错误

Question

我使用此代码处理Paypal支付数据传输（PDT）：

            $pp_hostname = "www.sandbox.paypal.com"; // Change to www.sandbox.paypal.com to test against sandbox
        // read the post from PayPal system and add 'cmd'
        $req = 'cmd=_notify-synch';

        $tx_token = $_GET['tx'];
        $auth_token = "my_token";
        $req .= "&tx=$tx_token&at=$auth_token";

        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, "https://$pp_hostname/cgi-bin/webscr");
        curl_setopt($ch, CURLOPT_POST, 1);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $req);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 1);
        //set cacert.pem verisign certificate path in curl using 'CURLOPT_CAINFO' field here,
        //if your server does not bundled with default verisign certificates.
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
        curl_setopt($ch, CURLOPT_HTTPHEADER, array("Host: $pp_hostname"));
        $res = curl_exec($ch);
        curl_close($ch);

        if(!$res){
            echo "HTTP ERROR";
        }else{ ...

但是它不起作用，我的代码告诉我请求失败，并且它输出HTTP ERROR因为$ res为空。 为什么呢？