xpath.php

<?php

$xml = <<<XML
<users>
  <user>
    <name>tim</name>
    <email>tim@timland.com</email>
    <password>plainpasswordsarebad</password>
  </user>
</users>
XML;

$doc = new SimpleXMLElement($xml);
// dynamically generate an xpath like /users/user[name="tim"]
$users = $doc->xpath('/users/user[name="' . $username . '"]');

if (empty($users)) {
  // no user exists with that username
} else {
  $user = $users[0]; // if there's more than one element, you have duplicate users

  if ($user->password == $password) {
    // login success
  }
}

?>