为什么此xpath的值不为true?

我试图使用xpath评估一个XML节点,我不知道为什么它的评估结果不是真的。

XML

<?xml version="1.0"?>
<users>
    <user>
        <username>tom</username>
        <password>d644cd4b1c72f563855e689d46d9198e</password>
    </user>
    <user>
        <username>jeff</username>
        <password>smith</password>
    </user>
</users>

当我提交表单时,此脚本被调用

    <?php
        //needed for firePHP in firebug
        include('FirePHPCore/fb.php');
        ob_start();

        $error = false;
        if(isset($_POST['login'])) {
            $username = preg_replace('/[^A-Za-z0-9]/', '', $_POST['username']);
            $password = md5($_POST['password']);


            if(file_exists("../users.xml")) {

                $xmlobject = simplexml_load_file("../users.xml");
                fb("username is: ".$username); //returns tom
                fb($xmlobject->xpath("//*[username='tom']")); //returns the entire array of elements. How do i make it return just the node value?

                //why does this evaluate to false?
                if($username == $xmlobject->xpath("//*[username='tom']")) {
                    fb("got here");
                } else {
                    fb("got here instead");
                }   
            }
            $error = true;
 }
?>