XML : xml user database exist

i try to make an user database with xml. Now i don't know how to do this. This is my code

php:

  <?php  session_start();      if($_SERVER['REQUEST_METHOD'] == 'POST'){  if($_POST['user'] == ""){  echo "this field may not empty";  }else{  $user = $_POST['user'];  }  ?>     

How can i check if user is already exist in the xml file? And if username or userid is not exist create a new one

xml:

  <users>  <user>  <userid>1</userid>  <username>usera</username>  </user>  <user>  <userid>2</userid>  <username>userb</username>  </user>  etc...  </users>