It doesn't seem to be documented anywhere, but you can refer to an element "value" for the purpose of changing it like so:
<?php
$xml = simplexml_load_string('<root><number>1</number></root>');
echo $xml->asXml(). "\n\n";
$xml->number->{0} = $xml->number->{0} + 1;
echo $xml->asXml();
?>
echos:
<?xml version="1.0"?>
<root><number>1</number></root>
<?xml version="1.0"?>
<root><number>2</number></root>
However, this only works with a direct assignment, not with any of the other operators:
<?php
$xml = simplexml_load_string('<root><number>1</number></root>');
echo $xml->asXml(). "\n\n";
$xml->number->{0} += 1;
$xml->number->{0}++;
echo $xml->asXml();
?>
Both of the above cases would result in:
<?xml version="1.0"?>
<root><number>1</number></root>
<?xml version="1.0"?>
<root><number>1<0/></number></root>
