How can I forward $_POST with PHP and cURL?

Perhaps:

curl_setopt($ch, CURLOPT_POSTFIELDS, $_POST);

From curl_setopt:

This can either be passed as a urlencoded string like 'para1=val1&para2=val2&...' or as an array with the field name as key and field data as value.

Do this,

curl_setopt($handle, CURLOPT_POSTFIELDS, http_build_query($_POST));