PHP 4: Class problem

[nebulom]

Anyone knows why I am getting 'eeek' with this code

class test {
  var $a;
  var $b;
  
  function x() {
    $this->a = "hello";
  }
  
  function y() {
    $this->b = "world";
  }
  
  function z() {
    echo $this->b;
  }
}

$t = new test();
$t->z() or die('eeek');
?>

Thanks in advance.