I guess you do not need cURL to redirect the browser, PHP can do that.
You are posting something => getting a response from the destination => doing the redirect
I have a similar application posting something with curl to a payment server, receiving an answer and then redirecting the customer there.
STEP 1: use of curl
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,"https://<paymentsite>");
curl_setopt($ch, CURLOPT_POST,1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $xmltext);
curl_setopt($ch, CURLOPT_POSTFIELDSIZE, 0);
curl_setopt($ch, CURLOPT_TIMEOUT, 60);
curl_setopt($ch, CURLOPT_HEADER,0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
$result=curl_exec ($ch);
STEP 2: paring the $result, extracting an Session-ID and a simply PHP redirect follows.
header ("Location: https://<paymentsite>?id=".$CLIENTSESSID);