send cURL post to be received exactly like form post

Is it possible to send a few simple form entries using PHP in a separate file so that a remote server receives the data in the exact same way as if sent directly from the form? Here is the situation:

Users make entries into a form on my site which are then sent to a remote server which generates a variable length page of results for the users' perusal. I want the results to be displayed on my site but I discover that there is a problem with iframe height so I am trying PHP in a separate file to avoid an iframe. Then maybe I can cache the result pages and display them without needing an iframe. The Javascript remote server security issue looks daunting so I am trying to use a PHP solution first.

The following PHP code generates a "... no wrapper... " error and is apparently because fopen and fsockopen etc. are set to "off", for security reasons, by the host that I use:

$url="http://remote_server.cgi";
foreach($_POST as $key => $value)
{$url .="$key=" . urlencode($value) . "&";}
$array = file($url);]

PHP:

The host runs PHP version 4.3.11 and has cURL enabled so I try that but the form entries are not being accepted as they are with a post direct from the form:

$URL="http://remote_server.cgi";
$postfields=array();        //FOR urlencode and querystring expected by CURL_POSTFIELDS. 
foreach($_POST as $key=>$val) $postfields[]="$key=".urlencode($val);
$postfields = join("&", $postfields);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $URL); 
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $_POST);
curl_exec ($ch);    
curl_close ($ch);

PHP:

In case useragent recognition was a security issue at the remote server, I added:

curl_setopt($ch, CURLOPT_USERAGENT, 'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.13) Gecko/20080311 Firefox/2.0.0.13');

PHP:

I know all entries are received by using:

print_r($_POST);

PHP:

But the remote server still says "Error in form found. You are not authorized... etc. etc... "

The remote server must be receiving the entries differently from how it gets them directly from the form.

What could the issues be and what fixes can I try?