xml xpath does not return node value

I have a test file where I'm trying to parse an xml string using SimpleXML's xpath method.

When I try to access a nodes values directly using xpath I get empty output, but when I use xpath to grab the elements and then loop through them it works fine.

When I look at the documentation, it seems like my syntax should work. Is there something I'm missing?

<?php

$xmlstring = '<?xml version="1.0" encoding="iso-8859-1"?>
<users>
  <user>
    <firstname>Sheila</firstname>
    <surname>Green</surname>
    <address>2 Good St</address>
    <city>Campbelltown</city>
    <country>Australia</country>
    <contact>
      <phone type="mobile">1234 1234</phone>
      <url>http://ift.tt/1yCJmXj;
      <email>pamela@example.com</email>
    </contact>
  </user>
  <user>
    <firstname>Bruce</firstname>
    <surname>Smith</surname>
    <address>1 Yakka St</address>
    <city>Meekatharra</city>
    <country>Australia</country>
    <contact>
      <phone type="landline">4444 4444</phone>
      <url>http://ift.tt/1yCJp5x;
      <email>bruce@yakka.example.com</email>
    </contact>
  </user>
</users>';


// Start parsing
if(!$xml = simplexml_load_string($xmlstring)){
    echo "Error loading string ";
} else {
    echo "<pre>";

    // Print all firstname values directly from xpath
    // This outputs the elements, but the values are blank
    print_r($xml->xpath("/users/user/firstname"));

    // Set a variable with all of the user elements and then loop through and print firstname values
    // This DOES output the values
    $users = $xml->xpath("/users/user");
    foreach($users as $user){
        echo $user->firstname;
    }

    // Find all firstname values by tag
    // This does not output the values 
    print_r($xml->xpath("//firstname"));
    echo "</pre>";
}