<?php
$s = '<?xml version="1.0" encoding="UTF-8"?>
<student>
<id>st01</id>
<name>Name 1</name>
</student>
';
$student = simplexml_load_string($s);
// email tag
$student->addChild('email', 'a@gmail.com');
// phone tag
$student->addChild('phone', '123456');
// address tag
$address = $student->addChild('address');
$address->addChild('street', 'street 1');
$address->addChild('ward', 'ward 1');
echo $student->asXML();
?>
<?xml version="1.0" encoding="UTF-8"?>
<student>
<id>st01</id>
<name>Name 1</name>
<email>a@gmail.com</email>
<phone>123456</phone>
<address>
<street>street 1</street>
<ward>ward 1</ward>
</address>
</student>