Ben bu php script kullanıyorum

$error = false;
if(isset($_POST['login'])){
    $username = preg_replace('/[^A-Za-z]/', '', $_POST['username']);
    $password = md5($_POST['password']);
    if(file_exists('users/' .$username . '.xml')){
	    $xml = new SimpleXMLElement('users/' .$username . '.xml', 0, true);
	    if($password == $xml->password){
		    session_start();
		    $_SESSION['username'] = $username;
		    header('Location: agis-employees.html');
		    die;
	    }
    }
    $error = true;
}

Ve ben bu hata ile sona

Fatal error: Cannot instantiate non-existent class: simplexmlelement in /home/virtual/site250/fst/var/www/html/employeeportal/index.php on line 7