防止save（）覆盖xml文件中的dtd

I'm writing a script that adds nodes to an xml file. In addition to this I have an external dtd I made to handle the organization to the file. However the script I wrote keeps overwriting the dtd in the empty xml file when it's done appending nodes. How can I stop this from happening?

Code:

<?php

/*Dom vars*/
$dom = new DOMDocument("1.0", "UTF-8");
$previous_value = libxml_use_internal_errors(TRUE);
$dom->load('post.xml');
libxml_clear_errors();
libxml_use_internal_errors($previous_value);
$dom->formatOutput = true;
$entry = $dom->getElementsByTagName('entry');
$date = $dom->getElementsByTagName('date');
$para = $dom->getElementsByTagname('para');
$link = $dom->getElementsByTagName('link');



/* Dem POST vars used by dat Ajax mah ziggen, yeah boi*/
if (isset($_POST['Text'])){
$text = trim($_POST['Text']);

}
/*
function post(){
    global $dom, $entry, $date, $para, $link, 
    $home, $about, $contact, $text;
*/  
    $entryC = $dom->createElement('entry');
    $dateC = $dom->createElement('date', date("m d, y H:i:s")) ;
    $entryC->appendChild($dateC);

$tab = "
";
$frags = explode($tab, $text);
$i = count($frags);
$b = 0;

while($b < $i){

$paraC = $dom->createElement('para', $frags[$b]);
$entryC->appendChild($paraC);
$b++;
}



$linkC = $dom->createElement('link', rand(100000, 999999));
$entryC->appendChild($linkC);
$dom->appendChild($entryC);
$dom->save('post.xml');

/*}

post();
*/echo 1;

?>

It looks like in order to do this, you'd have to create a DOMDocumentType using

DOMImplementation::createDocumentType 

then create an empty document using the DOMImplementation, and pass in the DOMDocumentType you just created, then import the document you loaded. This post: http://pointbeing.net/weblog/2009/03/adding-a-doctype-declaration-to-a-domdocument-in-php.html and the comments looked useful.

I'm guessing this is happening because after parsing/validation, the DTD isn't part of the DOM anymore, and PHP therefore isn't able to include it when the document is serialized.

Do you have to use a DTD? XML Schemas can be linked via attributes (and the link is therefore part of the DOM). Or there's RelaxNG, which can be linked via a processing instruction. DTDs have all this baggage that comes with them as a holdover from SGML. There are better alternatives.