XML : how to create a alert message for existing child node data in php

i have a xml document

  <root>     <info>        <name>John</name>        <address>USA</address>     </info>     <info>        <name>David</name>        <address>Australia</address>     </info>  </root>    

i need to check the names.. if the name John exists in the node then it should show alert that the name already exists and if not exists the data should append in the xml document..

sry for my english

any help..??

Thanks in advance...