Before:

<security>
    <id>15</id>
</security>

Change the ID:

<?php
$s = simplexml_load_file('security.xml');
echo $s->id;
$s->id = 20;
simplexml_save_file($s, 'security.new.xml');
? >

After:

<security>
    <id>20</id>
</security>