CURLOPT_POST should be set before CURLOPT_POSTFIELDS. Otherwise you might encounter 411 Length required error.

Following code generates "411 Length Required" on nginx/1.1.15
<?php
curl_setopt ($ch, CURLOPT_POSTFIELDS, $postfields); 
curl_setopt ($ch, CURLOPT_POST, 1);
?>

But this one works.

<?php
curl_setopt ($ch, CURLOPT_POST, 1);
curl_setopt ($ch, CURLOPT_POSTFIELDS, $postfields); 
?>