<?php
$xml = <<< XML
<foo>
    <bar>
        <baz bam="boom">baddam</baz>
    </bar>
</foo>
XML;

$foo = simplexml_load_string($xml);
$allBazElements = $foo->xpath('//baz');
echo
    $allBazElements[0],        // baddam
    $allBazElements[0]['bam'], // boom
    PHP_EOL;

$allBazElements[0][0] = 'changed';
$allBazElements[0]['bam'] = 'changed too';

echo $foo->asXml();

baddamboom
<?xml version="1.0"?>
<foo>
    <bar>
        <baz bam="changed too">changed</baz>
    </bar>
</foo>