Create a XML using DomDocument

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

//create the main tags, without values
$books = $dom->createElement('books');
$book_1 = $dom->createElement('book');

// create some tags with values
$name_1 = $dom->createElement('name', 'PHP - An Introduction');
$price_1 = $dom->createElement('price', '$5.95');
$id_1 = $dom->createElement('id', '1');

//create  and append an attribute
$attr_1 = $dom->createAttribute('version');
$attr_1->value = '1.0';
//append the attribute
$id_1->appendChild($attr_1);

//create the second tag book with different namespace
$namespace = 'www.example.com/libraryns/1.0';

//include the namespace prefix in the books tag
$books->setAttributeNS('http://www.w3.org/2000/xmlns/', 'xmlns:ns', $namespace);
$book_2 = $dom->createElementNS($namespace,'ns:book');
$name_2 = $dom->createElementNS($namespace, 'ns:name');

//create a CDATA section (that is another DOMNode instance) and put it inside the name tag
$name_cdata = $dom->createCDATASection('PHP - Advanced');
$name_2->appendChild($name_cdata);
$price_2 = $dom->createElementNS($namespace, 'ns:price', '$25.00');
$id_2 = $dom->createElementNS($namespace, 'ns:id', '2');

//create the XML structure
$books->appendChild($book_1);
$book_1->appendChild($name_1);
$book_1->appendChild($price_1);
$book_1->appendChild($id_1);
$books->appendChild($book_2);
$book_2->appendChild($name_2);
$book_2->appendChild($price_2);
$book_2->appendChild($id_2);

$dom->appendChild($books);

//saveXML() method returns the XML in a String
print_r ($dom->saveXML());

<?xml version="1.0" encoding="utf-8"?>
<books xmlns:ns="www.example.com/libraryns/1.0">
  <book>
    <name>PHP - An Introduction</name>
    <price>$5.95</price>
    <id version="1.0">1</id>
  </book>
  <ns:book>
    <ns:name><![CDATA[PHP - Advanced]]></ns:name>
    <ns:price>$25.00</ns:price>
    <ns:id>2</ns:id>
  </ns:book>
</books>