Description:
------------
I'm using libxml2 2.6.17.

In the following code, the call to createElement() changes the value of the private property $rootTag to null.

Reproduce code:
---------------
<?php

ini_set('display_errors', '1');
error_reporting(E_ALL | E_STRICT);

class bdmDOMTable extends DOMDocument
{
    private $rootTag;
    private $rowTag;

    public function __construct($rootTag, $rowTag)
    {
        parent::__construct('1.0', 'UTF-8');

        $this->preserveWhiteSpace = FALSE;
        $this->formatOutput = FALSE;

        if ($rootTag) {
            $this->rootTag = $rootTag;
        }
        else {
            $this->rootTag = 'table';
        }
        
        if ($rowTag) {
            $this->rowTag = $rowTag;
        }
        else {
            $this->rowTag = 'row';
        }

        $this->appendChild($this->createElement($this->rootTag));

    }

}

$modulesDOM = new bdmDOMTable('modules', 'module');

var_dump($modulesDOM);
?>

Expected result:
----------------
object(bdmDOMTable)#1 (2) { ["rootTag:private"]=>  string(7) "modules" ["rowTag:private"]=>  string(6) "module" }

Actual result:
--------------
object(bdmDOMTable)#1 (2) { ["rootTag:private"]=>  NULL ["rowTag:private"]=>  string(6) "module" }