joomla external sign in

I am trying to login to the main joomla site from a xenforo site in the subdirectory of the joomla site i.e. www.joomlasite.com/xenforo/index.php?login/login
i have writtent the following code to login to the site:
xenforo login modification:

url = 'http://www.joomlasite.com/xenforo_login.php';
    $fields = array(
                            'username' => urlencode($data['login']),
                            'password' => urlencode($data['password'])
                    );

    //url-ify the data for the POST
    foreach($fields as $key=>$value) { $fields_string .= $key.'='.$value.'&'; }
    rtrim($fields_string, '&');

    //open connection
    $ch = curl_init();

    //set the url, number of POST vars, POST data
    curl_setopt($ch,CURLOPT_URL, $url);
    curl_setopt($ch,CURLOPT_POST, count($fields));
    curl_setopt($ch,CURLOPT_POSTFIELDS, $fields_string);

    //execute post
    $result = curl_exec($ch);

    //close connection
    curl_close($ch);

and xenforo_login.php file in www.joomlasite.com/xenforo_login.php is as follows:

    define( '_JEXEC', 1 );
define('JPATH_BASE', dirname(__FILE__) );
define( 'DS', DIRECTORY_SEPARATOR );

require_once( JPATH_BASE .DS.'configuration.php');
require_once ( JPATH_BASE .DS.'includes'.DS.'defines.php' );
require_once ( JPATH_BASE .DS.'includes'.DS.'framework.php' );
require_once ( JPATH_BASE .DS.'libraries'.DS.'joomla'.DS.'factory.php' );

/* Create the Application */
$mainframe =& JFactory::getApplication('site');
jimport('joomla.plugin.helper');
jimport( 'joomla.user.user');
jimport( 'joomla.session.session');

$credentials = array();
$credentials['username'] = $_REQUEST['username'];
$credentials['password'] = $_REQUEST['password'];
//print_r($credentials);exit();

//perform the login action
$error = $mainframe->login($credentials);
$user = JFactory::getUser();

Now the issue I am facing is the code gives me the user but it does not login the user in the www.joomlasite.com nut the user is logged in www.joomlasite.com/xenforo
Can someone please help me asap!