php - DOMDocument cannot change parentNode

Solution:

Since the document has no parent node you need to append the root node directly to the document, like this:

$DOMDocument->appendChild($node);
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This works sinceDOMDocument extendsDOMNode.

Working example:

error_reporting(E_ALL);

function xml_encode($mixed, &$DOMDocument=null) {
    if (is_null($DOMDocument)) {
        $DOMDocument =new DOMDocument;
        $DOMDocument->formatOutput = true;
        xml_encode($mixed, $DOMDocument);
        return $DOMDocument->saveXML();
    } else {
        if (is_array($mixed)) {
            $node = $DOMDocument->createElement('urlset', 'hello');
            $DOMDocument->appendChild($node);
        }
    }   
}

$data = array();
for ($x = 0; $x <= 10; $x++) {
    $data['urlset'][] = array(
       'loc' => 'http://www.example.com/user',
       'lastmod' => 'YYYY-MM-DD',
       'changefreq' => 'monthly',
       'priority' => 0.5 
    );  
}

header('Content-Type: application/xml');
echo xml_encode($data);
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Btw, if you just want to serialize an XML file,DOM is a bit overhead. I would use a template engine for this, meaning handle it as plain text.

Solution:

This should work, when you create a new DOMDocument you don't have a root element yet, so you can just create it and add it to the document

//You could add this to the top of xml_encode
if($DOMDocument->parentNode === null) {
   $root = $DOMDocument->createElement('root');
   $root = $DOMDocument->appendChild($root);
}

//Your script working: 
<?php
error_reporting(E_ALL);

function xml_encode($mixed, $DOMDocument=null) {
    if (is_null($DOMDocument)) {
        $DOMDocument =new DOMDocument();
        $DOMDocument->formatOutput = true;

        //add here, but note that in the "else" it isn't sure if the DOMDocument has a root element 
        $root = $DOMDocument->createElement('root');
        $root = $DOMDocument->appendChild($root);
        xml_encode($mixed, $root);

        echo $DOMDocument->saveXML();
    } else {
        if (is_array($mixed)) {
            $node = $DOMDocument->createElement('urlset', 'hello');
            $DOMDocument->parentNode->appendChild($node);
        }
    }
}
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I'm not sure why you need the parentNode? you could do$DOMDocument->appendChild();