<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE Client [
<!ELEMENT  Client (Accounts)*>
<!ELEMENT Accounts (Group,Tag)>
<!ATTLIST Accounts Id  ID  #REQUIRED>
<!ELEMENT Group (#PCDATA)>
<!ELEMENT Tag (#PCDATA)>
]>
<Client>
    <Accounts Id="A1">
        <Group>0</Group>
        <Tag>自定义名称1</Tag>
    </Accounts>     
    <Accounts Id="A2">
         <Group>0</Group>
         <Tag>自定义名称2</Tag>
    </Accounts>     
</Client>

$domDoc = DOMDocument::loadXML($xml);
$rootNode = $domDoc->getElementsByTagName('Client')->item(0);

// 1.
$accounts = $domDoc->createElement('Accounts');
$accounts->setAttribute('Id', 'A3');
$group = $domDoc->createElement('Group', '1');
$tag = $domDoc->createElement('Tag', '自定义名称3');
$accounts->appendChild($group);
$accounts->appendChild($tag);
$rootNode->appendChild($accounts);

echo $domDoc->validate(), "\n";
echo $domDoc->saveXML();

// 3.
$A2 = $domDoc->getElementById('A2');
$A2->replaceChild(
  $domDoc->createElement('Group', '100'),
  $A2->getElementsByTagName('Group')->item(0)
);
$A2->replaceChild(
  $domDoc->createElement('Tag', '自定义名称200'),
  $A2->getElementsByTagName('Tag')->item(0)
);

echo $domDoc->validate(), "\n";
echo $domDoc->saveXML();

// 2.
$rootNode->removeChild($A2);

echo $domDoc->validate(), "\n";
echo $domDoc->saveXML();