XML/DOM - Can't update empty node value

The problem is:

$vPaymt->Name->nodeValue = self::NAME;

It is not related to DOM. Here is another source that reproduces the error message:

$o = new stdClass();
$o->PropertyName->SubPropertyName = 23;

Message:

Warning: Creating default object from empty value in /tmp/ex... on line 4

What happens is that $vPaymt is an object (a DOMElement), but $vPaymt->Name is a non existing property. You use that non existing property as an object by writing a property of it. PHP initializes it as a default object and warns you about that.

DOM has no autmatic mapping for child elements (like SimpleXML). You should use XPath to fetch the Name node.

$xml = <<<'XML'
<?xml version="1.0" encoding="UTF-8"?>
<Document>
    <Customer>
        <Payment>
            <Name></Name>
        </Payment>
    </Customer>
</Document>
XML;

$dom = new DOMDocument();
$dom->loadXml($xml);
$xpath = new DOMXPath($dom);

foreach ($xpath->evaluate('//Payment/Name') as $node) {
  $node->nodeValue = '';
  $node->appendChild($dom->createTextNode('FOO'));
}

echo $dom->saveXml();

Output:

<?xml version="1.0" encoding="UTF-8"?>
<Document>
    <Customer>
        <Payment>
            <Name>FOO</Name>
        </Payment>
    </Customer>
</Document>

WARNING!

Never write DOMNode::$nodeValue with a string from a variable. It does not fully escape the provided string. Setting an empty string is fine (no characters to escape). Always create a text node and append it to avoid broken XML.