Description:
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DOMElement#setAttribute() correctly handles setting the default (unprefixed) namespace, but does not handle prefixed namespaces, and simply creates a DOMAttr and appends it to the node.

Test script:
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$xml = '<?xml version="1.0" ?><test />';
$doc = new DOMDocument();
$doc->loadXML($xml);

$element = $doc->documentElement;

// Unprefixed works fine

echo "\nElement should not have namespace:\n";
var_dump($element->lookupPrefix('test:unprefixed'), $element->lookupNamespaceURI(null));

echo "\nsetAttribute() call should succeed:\n";
var_dump($element->setAttribute('xmlns', 'test:unprefixed'));

echo "\nElement should have xmlns attribute:\n";
var_dump($element->hasAttribute('xmlns'));

echo "\nElement should have namespace:\n";
var_dump($element->lookupPrefix('test:unprefixed'), $element->lookupNamespaceURI(null));

// Prefixed does not work

echo "\nElement should not have namespace:\n";
var_dump($element->lookupPrefix('test:prefixed'), $element->lookupNamespaceURI('test'));

echo "\nsetAttribute() call should succeed:\n";
var_dump($element->setAttribute('xmlns:test', 'test:prefixed'));

echo "\nElement should have xmlns attribute:\n";
var_dump($element->hasAttribute('xmlns:test'));

echo "\nElement should have namespace:\n";
var_dump($element->lookupPrefix('test:prefixed'), $element->lookupNamespaceURI('test'));


Expected result:
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Namespace is registered with prefix

Actual result:
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DOMAttr is added to node but namespace is not registered