Error code while trying to use private variables in a function

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();

You don't define class properties (public/private/etc) in functions/methods. You do it in the body of the class.

class ac
{
    private $plain_username;
    private $md5_password;

    public function authentication()
    {
        $this->plain_username = $_POST['username'];
        $this->md5_password = md5($_POST['password']);
    }
}

//declare a class outside the class
$ac = new ac();

If you want to define variables in a function/method, just declare them without the public/private/protected

$plain_username = $_POST['username'];