To avoid multiple xmlns re-declaration, make sure you appending ElementNS into actual DOMDocument tree (not into some currently-assembed derelict element).
(PHP 5, PHP 7, PHP 8)
DOMDocument::createElementNS — Create new element node with an associated namespace
$namespace, string $qualifiedName, string $value = ""): DOMElement|falseThis function creates a new element node with an associated namespace. 此节点出现在文档中，除非是用诸如 DOMNode->appendChild() 等函数来将其插入。
namespaceThe URI of the namespace.
qualifiedName
The qualified name of the element, as prefix:tagname.
valueThe value of the element. By default, an empty element will be created. You can also set the value later with DOMElement::$nodeValue.
The new DOMElement or false if an error occurred.
DOM_INVALID_CHARACTER_ERR
Raised if qualifiedName contains an invalid character.
DOM_NAMESPACE_ERR
Raised if qualifiedName is a malformed qualified
name.
示例 #1 Creating a new element and inserting it as root
<?php
$dom = new DOMDocument('1.0', 'utf-8');
$element = $dom->createElementNS('http://www.example.com/XFoo', 'xfoo:test', 'This is the root element!');
// We insert the new element as root (child of the document)
$dom->appendChild($element);
echo $dom->saveXML();
?>以上示例会输出：
<?xml version="1.0" encoding="utf-8"?> <xfoo:test xmlns:xfoo="http://www.example.com/XFoo">This is the root element!</xfoo:test>
示例 #2 A namespace prefix example
<?php
$doc = new DOMDocument('1.0', 'utf-8');
$doc->formatOutput = true;
$root = $doc->createElementNS('http://www.w3.org/2005/Atom', 'element');
$doc->appendChild($root);
$root->setAttributeNS('http://www.w3.org/2000/xmlns/' ,'xmlns:g', 'http://base.google.com/ns/1.0');
$item = $doc->createElementNS('http://base.google.com/ns/1.0', 'g:item_type', 'house');
$root->appendChild($item);
echo $doc->saveXML(), "\n";
echo $item->namespaceURI, "\n"; // Outputs: http://base.google.com/ns/1.0
echo $item->prefix, "\n"; // Outputs: g
echo $item->localName, "\n"; // Outputs: item_type
?>以上示例会输出：
<?xml version="1.0" encoding="utf-8"?> <element xmlns="http://www.w3.org/2005/Atom" xmlns:g="http://base.google.com/ns/1.0"> <g:item_type>house</g:item_type> </element> http://base.google.com/ns/1.0 g item_type