Parsing the xml file to get clild node value

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PHP

i have an xml file

<?xml version="1.0"?>
  <user>
    <user_name>abc</user_name>
    <is_active>0</is_active>
 </user>

Now i want to check that weather the user is active or not from user name. How to get out of this? How to check in xml file for perticular username and then after user is acive or not? Can any one help me out?
Thanks in advance.