I get an error that says

Parse error: syntax error, unexpected T_PRIVATE in E:\PortableApps\xampp\htdocs\SN\AC\ACclass.php on line 6

while trying to run my script. I'm new to classes in PHP and was wondering if someone could point out my error. Here's the code for that part.

<?php
class ac
  {
  public function authentication()
    {
    private $plain_username = $_POST['username'];
    private $md5_password = md5($_POST['password']);

    $ac = new ac();