<?php
$s = '<?xml version="1.0" encoding="UTF-8"?>
<student>
<id>st01</id>
<name>Name 1</name>
</student>
';
$student = simplexml_load_string($s);
$dob = $student->addChild('dob', '20/10/2021');
$dob->addAttribute('pattern', 'dd/MM/yyyy');
$dob->addAttribute('place', 'Place 1');
echo $student->asXML();
?>
Output
<?xml version="1.0" encoding="UTF-8"?>
<student>
<id>st01</id>
<name>Name 1</name>
<dob pattern="dd/MM/yyyy" place="Place 1">20/10/2021</dob>
</student>