class User
{
	private $username;
	private $password;
	private $id;

	public __construct() {
		$this->username = "";
		$this->password = "";
		$this->id = 0;
	}

	public function IsValid()
	{
		if($id != 0) return true;
		else return false;
	}

	public function LoadByLogin($username, $password)
	{
		// $sql, populate id if valid
		return $this->IsValid();
	}

	public function LoadByID($id)
	{
		// unMD5 $id, $sql, then set id if valid
		return $this->IsValid();
	}
}

class Admin
{
	private $user;
	private $authenticated;

	public __construct()
	{
		$user = new User();
		$authenticated = "";
	}

	public function IsAuthenticated()
	{
		if($this->authenticated == "") $this->Authenticate();
		return $this->authenticated;
	}

	public function Authenticate()
	{
		if(isset($_SESSION['userid']))
			$this->authenticated =  // unhash md5 and pass to $this->user->LoadByID();
		elseif(isset($_POST['username'], $_POST['password'])
		{
			$this->authenticated =  $this->user->LoadByLogin($_POST['username'], $_POST['password']);
			if($this->authenticated) $_SESSION['userid'] = md5($user->id);
		}
		else
			$this->authenticated =  false;
	}
}

$admin = new Admin();
if($admin->IsAuthenticated())
{
	Header("Login: admin.php");
}
else
{
	// Login form
}