session.cookie_domain = ".example.com"

php_value session.cookie_domain .example.com

ini_set('session.cookie_domain', '.example.com' );

if(isset($_COOKIE['session_id']))
            session_id($_COOKIE['session_id']);
        Zend_Session::start(); //or session_start();
        if(!isset($_COOKIE['session_id']))
            setcookie('session_id', session_id(), 0, '/', '.yourdomain.com');

suhosin.session.cryptdocroot=Off
suhosin.cookie.cryptdocroot=Off

if(isset($_COOKIE['session_id']))
    session_id($_COOKIE['session_id']);
    Zend_Session::start(); //or session_start();

    if(!isset($_COOKIE['session_id']))
        setcookie('session_id', session_id(), 0, '/', '.yourdomain.com');