Keeping the cart information when login and logging out

[eluser]earlyriser[/eluser]
Here is the login function from SimpleSecureLogin. When the user logs, the session is destroyed and a new one is created.

If I delete the lines of DESTROY OLD SESSION and CREATE FRESH SESSION I can conserve the cart, but I want to know which security issues this could bring.

Code:

function login($user_email = '', $user_pass = '')
    {
        $this->CI =& get_instance();

        if($user_email == '' OR $user_pass == '')
            return false;


        //Check if already logged in
        if($this->CI->session->userdata('user_email') == $user_email)
            return true;
        
        
        //Check against user table
        $this->CI->db->where('user_email', $user_email);
        $query = $this->CI->db->getwhere($this->user_table);

        
        if ($query->num_rows() > 0)
        {
            $user_data = $query->row_array();

            $hasher = new PasswordHash(PHPASS_HASH_STRENGTH, PHPASS_HASH_PORTABLE);

            if(!$hasher->CheckPassword($user_pass, $user_data['user_pass']))
                return false;

            //Destroy old session
            $this->CI->session->sess_destroy();
            
            //Create a fresh, brand new session
            $this->CI->session->sess_create();

            $this->CI->db->simple_query('UPDATE ' . $this->user_table  . ' SET user_last_login = NOW() WHERE user_id = ' . $user_data['user_id']);

            //Set session data
            unset($user_data['user_pass']);
            $user_data['user'] = $user_data['user_email']; // for compatibility with Simplelogin
            $user_data['logged_in'] = true;
            $this->CI->session->set_userdata($user_data);
            
            return true;
        }
        else
        {
            return false;
        }    

    }