If you are doing a POST, and the content length is 1,025 or greater, then curl exploits a feature of http 1.1: 100 (Continue) Status.
See http://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html#sec8.2.3
* it adds a header, "Expect: 100-continue".
* it then sends the request head, waits for a 100 response code, then sends the content
Not all web servers support this though. Various errors are returned depending on the server. If this happens to you, suppress the "Expect" header with this command:
<?php
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Expect:'));
?>
See http://www.gnegg.ch/2007/02/the-return-of-except-100-continue/
