php对象转数组方法

<?php
class Point2D {
 var $x, $y;
 var $label;

 function Point2D($x, $y)
 {
 $this->x = $x;
 $this->y = $y;
 }

 function setLabel($label)
 {
 $this->label = $label;
 }

 function getPoint()
 {
 return array("x" => $this->x,
 "y" => $this->y,
 "label" => $this->label);
 }
}

// "$label" is declared but not defined
$p1 = new Point2D(1.233, 3.445);
print_r(get_object_vars($p1));

$p1->setLabel("point #1");
print_r(get_object_vars($p1));

?>

 Array
 (
 [x] => 1.233
 [y] => 3.445
 [label] =>
 )

 Array
 (
 [x] => 1.233
 [y] => 3.445
 [label] => point #1
 )