Paypal IPN 获得空白确认(应为“已验证"或“无效")

The response is supposed to be received as "VERIFIED" or "INVALID", and these are the only 2 possible responses... so what's going on =S. Any help is greatly appreciated.

完整代码如下:

$ipn_post_data = $_POST;

$response = "";

    // Choose url
    $url = 'https://www.sandbox.paypal.com/cgi-bin/webscr';

    // Set up request to PayPal
    $request = curl_init();
    curl_setopt_array($request, array
    (
        CURLOPT_URL => $url,
        CURLOPT_POST => TRUE,
        CURLOPT_POSTFIELDS => http_build_query(array('cmd' => '_notify-validate') + $ipn_post_data),
        CURLOPT_RETURNTRANSFER => TRUE,
        CURLOPT_HEADER => FALSE,
        CURLOPT_SSL_VERIFYPEER => TRUE,
        CURLOPT_CAINFO => 'cacert.pem',
    ));

    // Execute request and get response and status code
    $response = curl_exec($request);
    $status   = curl_getinfo($request, CURLINFO_HTTP_CODE);

    // Close connection
    curl_close($request);


$fh = fopen( "ipntest.txt", 'a+' );
$date = date( "Y-M-j H:i" );
fwrite( $fh, $date . " Response: " . $response . "
" );
fclose( $fh );

if($status == 200 && $response == 'VERIFIED')
{
    // All good! Proceed...
}
else
{
    // Not good. Ignore, or log for investigation...
}

文本文件输出:

2012-Nov-26 23:24 Response: 
2012-Nov-26 23:25 Response: 

我之前一直在使用这个代码，我一直在努力让它工作一个星期，所以这不仅仅是暂时的失败或其他什么......

[27-Nov-2012 21:46:11 UTC] cURL error: [77] error setting certificate verify locations:
  CAfile: /etc/ssl/certs/api_cert_chain.crt
  CApath: /etc/ssl/certs