附加到xml而不覆盖数据

Question

我刚开始学习xml，我现在有任务，从php提交的表单中获取数据并将其写入xml文件。我现在在类中有函数，在表单提交后将数据写入xml文件，它看起来像这样

function writeFile () {
    $dom = new DomDocument('1.0');
    $dom->preserveWhiteSpace = false;
    $dom->formatOutput = true;
    if( $xml = file_get_contents( 'include/users.xml') ) {
        $dom->loadXML( $xml, LIBXML_NOBLANKS );

         // find the headercontent tag
        $users = $dom->getElementsByTagName('users')->item(0);

        // добавление корня - <users> 
        //$users = $dom->appendChild($dom->createElement('users')); 
        $user = $users->appendChild($dom->createElement('user'));
        $xml_name = $user->appendChild($dom->createElement('name'));
        $xml_name->appendChild($dom->createTextNode($this->name));
        $xml_email = $user->appendChild($dom->createElement('mail'));
        $xml_email->appendChild($dom->createTextNode($this->email));

        // add the user tag before the first element in the <users> tag
        $users->insertBefore( $user, $users->firstChild );

        $dom->save('include/users.xml');
    }
}

然后在我的users.xml上我有：

<users>
  <user>
    <name>User1</name>
    <mail>new2@mail.ru</mail>
  </user>
</users>

但我的问题是我的<user>数据一直覆盖而不是追加新用户：

<users>
  <user>
    <name>User1</name>
    <mail>new2@gmail.com</mail>
  </user>
  <user>
    <name>User2</name>
    <mail>new3@gmail.com</mail>
  </user>
</users>

我会感激任何帮助