php5 + xml nodes

Xml example
 --------------------------------
<?xml version="1.0"?>
<config>
<network defaultroute="" dnsserver="">
<interface name="lan" type="ethernet" device="eth0"
ip="192.168.1.1" subnet="24"/>
<interface name="wan" type="ethernet" device="eth1"
ip="192.168.1.1" subnet="24"/>
<alias number="1" desc="teste" device="eth0"/>
<alias number="2" desc="teste1" device="eth1"/>
</network>
</config>
 --------------------------------- 

Hello all ... 

I'm working with simplexml (php5) to manipulate the data in
xml file, but I'm having one problem.
I need manipulate the nodes (add and remove) .
example
I need add more one node
<interface name="dmz" type="ethernet" device="eth3" ip="xxxxxxx"
subnet="24"/> 

and remove the node 

<alias number="2" desc="teste1" device="eth1"/> 

but I don't have idea ...
somebody can help me ?
Any example ... 

greats 

Carlos