XML feed google shopping

We already have a working XML that we make available on a link for Google Merchant Center to use (for a webshop in gardening products). In short it looks like this:

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://ift.tt/r5ut6F" xmlns:g="http://ift.tt/15FVsV8">
    <title>Companyname product data feed</title>
    <link rel="self" href="http://igarden.nl"/>
    <updated>2014-07-08 08:23:18</updated>
    <author>
        <name>Our companyname</name>
    </author><id>tag:companyname.nl,2014-07-08</id>
    <entry>
        <id>1001</id>
        <title>Productname</title>
        <link href="http://companyname.nl/"/>
        <g:price>123,10</g:price>
        <description>omschrijving van het product</description>
        <g:condition>new</g:condition>
        <g:brand>acd</g:brand>
        <g:mpn>12321</g:mpn>
        <g:ean>21353532235</g:ean>
        <g:image_link>http://ift.tt/1kyLcAa;
        <g:product_type>Huis &amp; Tuin &gt; category</g:product_type>
        <g:availability>in stock</g:availability>
        <g:shipping>
            <g:country>NL</g:country>
            <g:service>Standaard</g:service>
            <g:price>6,50</g:price>
        </g:shipping>
        <g:manufacturer>acd2</g:manufacturer>
        <g:weight>100g</g:weight>
        <g:featured_product>0</g:featured_product>
        <g:size>20x20x20</g:size>
    </entry>
</feed>

We're remaking the webshop, so I wanted to try to make it with DOMDocument, but I find it difficult. This is what I have so far in code:

/* create a dom document with encoding utf8 */
$domtree = new DOMDocument('1.0', 'UTF-8');

/* create the root element of the xml tree */
$xmlRoot = $domtree->createElement("feed");
$xmlRoot = $domtree->appendChild($xmlRoot);

$addProduct = $domtree->createElement("entry");
$addProduct = $xmlRoot->appendChild($addProduct);
$addProduct->appendChild($domtree->createElement('id','100123'));
$addProduct->appendChild($domtree->createElement('title','Blokhut'));
$addProduct->appendChild($domtree->createElement('g:price','123,50'));
$addProduct->appendChild($domtree->createElement('description','omschrijving van de blokhut'));
$addProduct->appendChild($domtree->createElement('g:condition','new'));

/* get the xml printed */
header("Content-Type: text/plain");
$domtree->formatOutput = true;
echo $domtree->saveXML();

Which outputs:

<?xml version="1.0" encoding="UTF-8"?>
<feed>
  <entry>
    <id>100123</id>
    <title>Blokhut</title>
    <g:price>123,50</g:price>
    <description>omschrijving van de blokhut</description>
    <g:condition>new</g:condition>
  </entry>
</feed>

So I got the basics working, but I have no clue how to make some elements working. How do I make these rows:

• Add this part to <feed>: <feed xmlns="http://ift.tt/r5ut6F" xmlns:g="http://ift.tt/15FVsV8">


• <title>Compayname product data feed</title>


• <link href="http://companyname.nl/"/>


• The <g:shipping> sub part

If I know how to make these, I think I can fill in the rest of the XML. I've tried so many things I found on here and by Googling, but I can't get them to work.