How do you rename a tag in SimpleXML through a DOM object?

Here's what's probably the simplest way to copy a node's children and attributes without using XSLT:

function clonishNode(DOMNode $oldNode, $newName, $newNS = null)
{
    if (isset($newNS))
    {
        $newNode = $oldNode->ownerDocument->createElementNS($newNS, $newName);
    }
    else
    {
        $newNode = $oldNode->ownerDocument->createElement($newName);
    }

    foreach ($oldNode->attributes as $attr)
    {
        $newNode->appendChild($attr->cloneNode());
    }

    foreach ($oldNode->childNodes as $child)
    {
        $newNode->appendChild($child->cloneNode(true));
    }

    $oldNode->parentNode->replaceChild($newNode, $oldNode);
}

Which you can use this way:

$dom = new DOMDocument;
$dom->loadXML('<foo><bar x="1" y="2">x<baz/>y<quux/>z</bar></foo>');

$oldNode = $dom->getElementsByTagName('bar')->item(0);
clonishNode($oldNode, 'BXR');

// Same but with a new namespace
//clonishNode($oldNode, 'newns:BXR', 'http://newns');

die($dom->saveXML());

It will replace the old node with a clone with a new name.

Attention though, this is a copy of the original node's content. If you had any variable pointing to the old nodes, they are now invalid.

Maybe easier way would be to replace the tags using preg functions for the XML source string?

Cleaner way

Create XSLT XML transformation file and use xsl PHP extension to translate it.

For this see this answer – Rename nodes with XSLT. PHP code part could be found in PHP documentation.