Log in with cURL and echo response with cookie

Log in with cURL and echo response with cookie

I am trying to write an "auto-login" script, that uses curl to log in to a
specific site and then echoing the response, so that the user can "bypass"
the login. (For example, to grant one-time logins etc)
Obviously, using the following method, the cookie is stored on the server,
not on the client, in cookie.txt.
I've tried setting $_COOKIE, but that won't work, since the URLs (actually
subdomains) of the auto-login script and the service to be logged into,
are different.
Login works fine. You see the screen as if you were logged in, but
clicking any link requires you to login again.
Any ideas? Thanks
The code I've used to auto-login:
<?php
$username="email@domain.com";
$password="password";
$url="http://sub.domain-to-login.com/index.php/sessions/login";
$cookie="cookie.txt";
$postdata = "email=".$username."&password=".$password.'&btn_login=Login';
$ch = curl_init();
curl_setopt ($ch, CURLOPT_URL, $url);
curl_setopt ($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
curl_setopt ($ch, CURLOPT_USERAGENT, "Mozilla/5.0 (Windows; U; Windows NT
5.1; en-US; rv:1.8.1.6) Gecko/20070725 Firefox/2.0.0.6");
curl_setopt ($ch, CURLOPT_TIMEOUT, 60);
curl_setopt ($ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt ($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt ($ch, CURLOPT_COOKIEJAR, $cookie);
curl_setopt ($ch, CURLOPT_REFERER, $url);
curl_setopt ($ch, CURLOPT_POSTFIELDS, $postdata);
curl_setopt ($ch, CURLOPT_POST, 1);
$result = curl_exec ($ch);
curl_close($ch);
echo $result;
?>