Tienes que hablar directamente con el script usando sockets y escribiendo la informacion que necesitas, algo asi:
Código PHP:
$postVars = array(
"variable1" => "valor",
"variable2" => "valor2"
);
$host = "dir.ip.del.server"; // o su dominio
$path = "/path/al/script.php.o.jsp.o.lo.que.sea";
$data = http_build_query( $postVars );
$fh = fsockopen($host, 80, $errorNumber, $errorString);
$request = "POST ".$path." HTTP/1.1\r\n";
$request.= "Host: ".$host."\r\n";
$request.= "User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1) Gecko/20061010 Firefox/2.0\r\n";
$request.= "Content-Type: application/x-www-form-urlencoded\r\n";
$request.= "Content-Length: ".strlen($data)."\r\n";
$request.= "Connection: close\r\n\r\n";
$request.= $data;
fwrite( $fh, $request );
$response = "";
while( !feof( $fh ) ) {
$response .= fread( $fh, 1024 );
}
fclose( $fh );
echo "Servidor respondio: " . $response;