curl sending GET instead of POST
When troubleshooting the entire class of PHP-cURL-related problems, you simply have to turn on CURLOPT_VERBOSE and give CURLOPT_STDERR a file handle.
tail -f your file, compare the headers and response to the ones you see in Firebug, and the problem should become clear.
The request is made from the server, and will not show up in Firebug. (You probably confused it with another request by your browser). Use wireshark to find out what really happens. You are not setting CURLOPT_FOLLOWLOCATION; redirects should not be followed.
Summarizing: Guess less, post more. Link to a pcap dump, and we will be able to tell exactly what you're doing wrong; or post the exact output of the php script, and we might.
The shown code does a multipart formpost (since you pass a hash array to the POSTFIELDS option), which probably is not what the target server expects.