PHP XPath query error after changing DOM - php

Question

I always receive the message PHP Notice: Trying to get property of non-object in ... when trying to XPath-query a newly created node.
My XML file looks like this:
<products xmlns='http://example.com/products'>
<product id='1'>
<name>Product name</name>
</product>
</products>
My PHP file essentially applies an XPath query to get the existing <product> and a second query for its <name>. This works fine.
Then I insert a new <product> with the child <name> to the DOM root element and try the second query on the newly created elements. Getting the attribute works fine, but the second query, which should get the first child <name>'s value, fails with the PHP Notice Trying to get property of non-object in ....
$xmlFile = __DIR__ . '/products.xml';
$xml = new DOMDocument();
$xml->load($xmlFile);
$xml->formatOutput = true;
$xpath = new DOMXPath($xml);
$xpath->registerNamespace('p', $xml->lookupNamespaceUri($xml->namespaceURI));
/*
* query the first product's ID and name
*/
$product1 = Product::$xpath->query("//p:product[#id=1]")->item(0);
$product1Id = $product1->attributes->getNamedItem('id')->nodeValue;
// => "1"
$product1Name = $xpath->query("p:name", $product1)->item(0)->nodeValue;
// => "Product name"
/*
* create the second product
*/
$product2Node = $xml->createElement('product');
$product2Node->setAttribute('id', '2');
$product2NameNode = $xml->createElement('name', 'Test');
$product2Node->appendChild($product2NameNode);
$product2 = $xml->documentElement->appendChild($product2Node);
/*
* query the second product's ID and name
*/
$product2Id = $product2->attributes->getNamedItem('id')->nodeValue;
// => "2"
$product2Name = $xpath->query("p:name", $product2)->item(0)->nodeValue;
// => PHP Notice: Trying to get property of non-object in ...
$xml->save($xmlFile);
After running the PHP file, the XML looks correct:
<products xmlns='http://example.com/products'>
<product id='1'>
<name>Product name</name>
</product>
<product id='2'>
<name>Test</name>
</product>
</products>
I am really stuck on this, I tried to save the XML before querying, reloading the XML after saving, recreating the XPath object, etc.

Answer 1

I believe you need to use the createElementNS function (http://php.net/manual/en/domdocument.createelementns.php; you may also want to check on setAttributeNS -- http://www.php.net/manual/en/domelement.setattributens.php) instead of createElement in order to explicitly indicate that these elements belong to the http://example.com/products namespace.
$product2Node = $xml->createElementNS('http://example.com/products', 'product');
$product2Node->setAttribute('id', '2');
$product2NameNode = $xml->createElementNS('http://example.com/products', 'name', 'Test');
$product2Node->appendChild($product2NameNode);
(It's a little surprising that reloading the XML after saving did not resolve this, but without seeing the code that attempted to do the reload it's difficult to know what might have gone wrong.)