PHP Simple object oriented application

I am getting undefined variable for periods and subPeriods on the last line of this program. not sure what the problem is. Could it be my instances?

This is my first proper attempt at oop in PHP so i am sure i am doing something wrong.

$global_periods = 5;
$global_subperiods = 2;

$questionslist = array("q_1_1", "q_1_2", "q_2_1", "q_2_2", "q_3_1", "q_4_1", "q_5_1");

class User {
    public $userId;
    public $periods = array();

    public function __construct($number)
    {
        $this->userId = $number;
    }

    public function addPeriod($pno)
    {
        $periods[] = new Period($pno);
    }
}

class Period {
    public $periodNo;
    public $subPeriods = array();

    public function __construct($number)
    {
        $this->periodNo = $number;
    }

    public function addSubPeriod($spno)
    {
        $subPeriods[] = new SubPeriod($spno);
    }
}

class SubPeriod {
    public $SubPeriodNo;
    public $answers = array();

    public function __construct($number)
    {
        $this->SubPeriodNo = $number;
    }

    public function addAnswer($answer)
    {
        $answers[] = new Answer($answer);
    }
}

class Question {
    public $answer;

    public function __construct($ans)
    {
        $this->answer = $ans;
    }

    public function getAnswer()
    {
        echo $answer;   
    }
}        

$userlist = array();

$sql = 'SELECT user_ref FROM _survey_1_as GROUP BY user_ref ORDER BY user_ref ASC';
$result = mysql_query($sql);

while ($row = mysql_fetch_array($result))
{
    $userlist[] = new User($row['user_ref']);
}

for ($i = 0; $i >= count($userlist); $i++)
{
    for ($x = 1; $x > $global_periods; $x++)
    {
        $userlist[i]->addPeriod($x);

        for ($y = 1; $y > $global_subperiods; $y++)
        {
            $userlist[i]->$periods[x]->addSubPeriod($y);

            foreach($questionslist as $aquestion)
            {
                $sql = 'SELECT ' . $questionNumber . ' FROM _survey_1_as WHERE user_ref = ' .
                     $i . ' AND answer_sub_period = ' . $y . ' AND answer_period = ' . $x .''; 

                $result = mysql_query($sql);

                while ($row = mysql_fetch_array($result))
                {
                    $userlist[i]->$periods[x]->$subPeriods[y]->addAnswer($row[$questionNumber]);
                }
            }
        }
    }   
}

$userlist[3]->$periods[2]->$subPeriods[2]->getAnswer();