html - Sharing session data across domains with PHP

My user is logged in atsite_url.com on a CMS, however I am now developing a mobile version of the site, outside the CMS on a subdomainm.site_url.com and would like the session to be carried over.

I have included a file with all the CMS' functions like this:

<?php include_once'/home/flyeurov/public_html/core/codon.config.php';?>
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I can login fine on the mobile site:

<input type="hidden" name="mobileVersion" value="True">
<input type="hidden" name="redir" value="" />
<input type="hidden" name="action" value="login" />
<input class="login-btn" type="submit" name="submit" value="Log In" />
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The above is processed bylogin.php. However, CMS login redirects it to CMS' page, and I need the mobile version to redirect to a page in /mobile directory wherem.site_url.com subdomain points. Hence, inlogin.php (not to be confused withmobile/login.php - mainlogin.php is for the CMS). I did this, and it redirects tomobile/crew_center.php - a page it should redirect to on a mobile version.

if (isset($_POST['mobileVersion'])) {
                header('Location: http://m.site_url.com/crew_center.php');
            } else {
                $this->post->redir = str_replace('index.php/', '', $this->post->redir);
                header('Location: '.url('/'.$this->post->redir));
            }
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Maybe the above code has something to do with my problem.

My problem is that the session from the main CMS is not carried over to the mobile site. Inmobile/index.php I have this statement, which does not work. When logged in, the user should seecrew_center.php as it would be pointless to display the login form. At the moment, when the user is logged in on the CMS, the login page still appears on the mobile version.

However, if I drop the subdomain in the browser and typesite_url/mobile it will open the right pagecrew_center.php.

<?php include_once'/home/flyeurov/public_html/core/codon.config.php';

session_set_cookie_params(0, '/', 'site_url.com');
session_start();

if(Auth::LoggedIn())
{
    header("Location: crew_center.php");
} 
else
{
    header("Location: login.php");
}

?>
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How can I fix this, and how can I share session data from a main domain to a subdomain with the use of PHP?

codon.config.php - http://pastebin.com/BWcbddGG