class Users {
    //table
    protected $users = 'users';
        //columns
        protected $user_id = 'user_id';
        protected $username = 'user_name';
        protected $password = 'user_password';
}

class Emails extends Users {
    protected $emails = 'emails';
        protected $address = 'email_address';
        protected $confirm = 'email_confirmed';
        //the foreign key user_id is already defined in users

    function getUserEmail($username) {
        $join = junct([$this->users, $this->emails], $this->user_id);
        $sql = select($join, [$this->address], [$this->username]);
        $result = prepped_query($sql, $username);
        return $result;
    }
}

$x = new Emails();
$email = $x->getUserEmail($username);