Duplicate xml namespace declarations php DOMDocument - php

Question

I use PHP DOMDocument to generate xml. Sometimes namespaces are declared only on root element, which is intended behaviour, but sometimes no.
For example:
$xml = new DOMDocument('1.0', 'utf-8');
$ns = "http://ns.com";
$otherNs = "http://otherns.com";
$docs = $xml->createElementNS($ns, "ns:Documents");
$doc = $xml->createElementNS($otherNs, "ons:Document");
$innerElement = $xml->createElementNS($otherNs, "ons:innerElement", "someValue");
$doc->appendChild($innerElement);
$docs->appendChild($doc);
$xml->appendChild($docs);
$xml->formatOutput = true;
$xml->save("dom");
I expect:
<?xml version="1.0" encoding="UTF-8"?>
<ns:Documents xmlns:ns="http://ns.com" xmlns:ons="http://otherns.com">
<ons:Document>
<ons:innerElement>someValue</ons:innerElement>
</ons:Document>
</ns:Documents>
But got:
<?xml version="1.0" encoding="UTF-8"?>
<ns:Documents xmlns:ns="http://ns.com" xmlns:ons="http://otherns.com">
<ons:Document xmlns:ons="http://otherns.com">
<ons:innerElement>someValue</ons:innerElement>
</ons:Document>
</ns:Documents>
Why declaration of xmlns:ons="http://otherns.com" appears on Document element, but not in <innerElement>? And how to prevent duplicates?

Answer 1

It's very easy. Just add your nodes into document tree.
Additionally you can explicitly create xmlns:XXX atrtribute in root node.
See example:
namespace test;
use DOMDocument;
$xml = new DOMDocument("1.0", "UTF-8");
$ns = "http://ns.com";
$otherNs = "http://otherns.com";
$docs = $xml->createElementNS($ns, "ns:Documents");
$xml->appendChild($docs);
$docs->setAttributeNS('http://www.w3.org/2000/xmlns/', 'xmlns:ons', $otherNs);
$doc = $xml->createElement("ons:Document");
$docs->appendChild($doc);
$innerElement = $xml->createElement("ons:innerElement", "someValue");
$doc->appendChild($innerElement);
$xml->formatOutput = true;
echo $xml->saveXML();
Result:
<?xml version="1.0" encoding="UTF-8"?>
<ns:Documents xmlns:ns="http://ns.com" xmlns:ons="http://otherns.com">
<ons:Document>
<ons:innerElement>someValue</ons:innerElement>
</ons:Document>
</ns:Documents>