Sharing SESSION Between Multiple Subdomains

I'm using session like bellow in my CI app on a domain (i.e. domain.com)-

PHP Code:

$set['x'] = $id;
$set['y'] = $name;
$set['z'] = $lavel;
$this->session->set_userdata($set); 


Now I want to use these session variables on a subdomain (i.e. sub.domain.com). How to do that..? Is there any option in CI to do that?

Thank you in advance.

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