Adding to the DOCTYPE using PHP dom

I'm trying to create an XML document that looks something like this...

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE stylesheet  [
    <!ENTITY nbsp   "&#160;">
    <!ENTITY copy   "&#169;">
    <!ENTITY reg    "&#174;">
    <!ENTITY trade  "&#8482;">
    <!ENTITY mdash  "&#8212;">
    <!ENTITY ldquo  "&#8220;">
    <!ENTITY rdquo  "&#8221;"> 
    <!ENTITY pound  "&#163;">
    <!ENTITY yen    "&#165;">
    <!ENTITY euro   "&#8364;">
]>
<NewsPost>
    <Post>
        <PermaLink>http://ift.tt/1I7sGfB;
        <Title>Change of Venue for Monday Meetings until March 2015</Title>
        <Content>Due to building work at Eccleshill library, the Monday meetings will be held at     Upper Bolton Conservative Club, Idle Road, Bradford, BD2 4JN.&#13;
&#13;
&nbsp;&#13;
&#13;
&nbsp;&#13;
&#13;
&lt;span style="color: #ffff00"&gt;&lt;strong&gt;Update &lt;/strong&gt;&lt;/span&gt;&#13;
&#13;
The building work is taking longer than expected; however, we hope to be back at the Library by     the end of March 2015.</Content></Post></NewsPost>

I'm tring to do this using PHP. The current code that I have so far is this...

    $imp = new DOMImplementation;

    $dtd = $imp->createDocumentType('stylesheet', '', '');

    $domDoc = new DOMDocument('1.0', 'utf-8');
    $domDoc->preserveWhiteSpace = false;

    require_once(newsFolder.'/wp-blog-header.php'); 
    //global $post;
    $args = array( 'posts_per_page' => 1 );
    $myposts = get_posts( $args );


    $rootElement = $domDoc->createElement('NewsPost');
    $domDoc->appendChild($rootElement); 

    foreach( $myposts as $post ) : setup_postdata($post);
        $postNode = $domDoc->createElement("Post");
        $rootElement->appendChild($postNode);

        $permaLinkNode = $domDoc->createElement("PermaLink",get_permalink());
        $postNode->appendChild($permaLinkNode);

        $titleNode = $domDoc->createElement("Title",get_the_title());
        $postNode->appendChild($titleNode);

        //$contentNode = $domDoc->createElement("Excerpt",get_the_excerpt());
        //$postNode->appendChild($contentNode);

        $contentNode = $domDoc->createElement("Content",get_the_content());
        $postNode->appendChild($contentNode);
    endforeach;

    $domDoc->save(cacheFolder.'LatestWordPressEntry.xml');

    unset($domDoc);

You'll notice that there is no code to add the tags to the !DOCTYPE

I'm looking all over the net and can't see the best practice method of doing this. I really do not want to resort to saving the XML to a string, then doing a string replace (which is always a huge cludge)

Any help on this would be greatly appreciated.

Bascially, I'm looking to turn the

<!DOCTYPE stylesheet>

tag into

<!DOCTYPE stylesheet  [
    <!ENTITY nbsp   "&#160;">
    <!ENTITY copy   "&#169;">
    <!ENTITY reg    "&#174;">
    <!ENTITY trade  "&#8482;">
    <!ENTITY mdash  "&#8212;">
    <!ENTITY ldquo  "&#8220;">
    <!ENTITY rdquo  "&#8221;"> 
    <!ENTITY pound  "&#163;">
    <!ENTITY yen    "&#165;">
    <!ENTITY euro   "&#8364;">
]>