什么会导致PayPal'；s AdaptivePayments API返回；内部服务器错误.请查看服务器日志以了解

我正在集成PayPal的AdaptivePayment API，但每次我打电话时都会收到CURL呼叫返回的错误"内部服务器错误。请查看服务器日志了解详细信息"。我相信我已经具备了所有必需的标题。其他人有这个问题吗？我的代码如下：

            $appID = "APP-80W284485P519543T";
            $username = "***********";
            $password = "***********";
            $signature = "**********";
            $endpoint = "https://svcs.sandbox.paypal.com/AdaptivePayments/";
            $headers = array(
                "X-PAYPAL-SECURITY-USERID: " . $username,
                "X-PAYPAL-SECURITY-PASSWORD: ". $password,
                "X-PAYPAL-SECURITY-SIGNATURE: ". $signature,
                "X-PAYPAL-REQUEST-DATA-FORMAT: JSON",
                "X-PAYPAL-RESPONSE-DATA-FORMAT: JSON",
                "X-PAYPAL-APPLICATION-ID: ". $appID
            );
            $packet = array(
                "actionType" => "PAY",
                "currencyCode" => "USD",
                "cancelUrl" => "cancel.html",
                "returnUrl" => "success.html",
                "receiverList" => array(
                    "receiver" => array(
                        array(
                            "amount" => "1.00",
                            "email" => "example@gmail.com",
                            "primary" => true
                        ),
                        array(
                            "amount" => "0.50",
                            "email" => "example2@gmail.com",
                            "primary" => false
                        )
                    )
                ),
                "requestEnvelope" => array(
                    "errorLanguage" => "en_US"
                )
            );

            $ch = curl_init();
            curl_setopt($ch, CURLOPT_URL, $endpoint."PAY");
            curl_setopt($ch, CURLOPT_VERBOSE, 1);
            //turning off the server and peer verification(TrustManager Concept).
            curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
            curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);
            curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
            curl_setopt($ch, CURLOPT_POST, 1);
            curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
            curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($packet));
            $response = curl_exec($ch);
            if (curl_errno($ch)) 
            {
                // moving to display page to display curl errors
                echo curl_errno($ch);
                echo curl_error($ch);
                  //Execute the Error handling module to display errors. 
            } 
            else 
            {
                echo "RESPONSE";
                header('Content-Type: application/json');
                echo $response;
                curl_close($ch);
            }