<?php
 class DOMxDocument extends DOMDocument {
    public $X=22;
    public $Y = array(
       'a' => false,
       'b'=>true
    );

    public function __construct($newconfig=NULL) {
         print "X={$this->X}\nY:";
         var_dump($this->Y); // NULL!!
         parent::__construct("1.0", "UTF-8");
    }
 }

$d = new DOMxDocument();
?>

X=22
Y:array(2) {
  ["a"]=>
  bool(false)
  ["b"]=>
  bool(true)
}