PayPal "Security header is not valid" error. What to do with this?

Question

Hi there,

I am using PayPal's DoDirectPayment API for payment integration. I am getting an error message like below.

---

Security error:
Error no: 10002
Error message: Security header is not valid

---

Here is my code,

    $environment = 'sandbox';
    // Set up your API credentials, PayPal end point, and API version.
    $API_UserName = urlencode('xxxxxxxx');
    $API_Password = urlencode('xxxxxxxx');
    $API_Signature = urlencode('xxxxxxxxxxxxxxxxxxxxxxxxxxx');
    $API_Endpoint = "https://api-3t.paypal.com/nvp";
    if("sandbox" === $environment || "beta-sandbox" === $environment) {
        $API_Endpoint = "https://api-3t.$environment.paypal.com/nvp";
    }
    $version = urlencode('51.0');

    // setting the curl parameters.
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $API_Endpoint);
    curl_setopt($ch, CURLOPT_VERBOSE, 1);

    // Set the curl parameters.
    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);

    // Set the API operation, version, and API signature in the request.
    $nvpreq = "METHOD=$methodName_&VERSION=$version&PWD=$API_Password&USER=$API_UserName&SIGNATURE=$API_Signature$nvpStr_";

    // Set the request as a POST FIELD for curl.
    curl_setopt($ch, CURLOPT_POSTFIELDS, $nvpreq);

    // Get response from the server.
    $httpResponse = curl_exec($ch);

Please help me out from this problem.

Thanks.