Loging PHP

class Auth
{
	var $user_id;
	var $username;
	var $password;
	var $ok;
	var $salt = "34asdf34";
	var $domain = ".domain.com";
 
	function Auth()
	{
		global $db;
 
		$this->user_id = 0;
		$this->username = "Guest";
		$this->ok = false;
 
		if(!$this->check_session()) $this->check_cookie();
 
		return $this->ok;
	}
 
	function check_session()
	{
		if(!empty($_SESSION['auth_username']) && !empty($_SESSION['auth_password']))
			return $this->check($_SESSION['auth_username'], $_SESSION['auth_password']);
		else
			return false;
	}
 
	function check_cookie()
	{
		if(!empty($_COOKIE['auth_username']) && !empty($_COOKIE['auth_password']))
			return $this->check($_COOKIE['auth_username'], $_COOKIE['auth_password']);
		else
			return false;
	}
 
	function login($username, $password)
	{
		global $db;
		$db->query("SELECT user_id FROM users WHERE username = '$username' AND password = '$password'");
		if(mysql_num_rows($db->result) == 1)
		{
			$this->user_id = mysql_result($db->result, 0, 0);
			$this->username = $username;
			$this->ok = true;
 
			$_SESSION['auth_username'] = $username;
			$_SESSION['auth_password'] = md5($password . $this->salt);
			setcookie("auth_username", $username, time()+60*60*24*30, "/", $this->domain);
			setcookie("auth_password", md5($password . $this->salt), time()+60*60*24*30, "/", $this->domain);
 
			return true;
		}
		return false;
	}		
 
	function check($username, $password)
	{
		global $db;
		$db->query("SELECT user_id, password FROM users WHERE username = '$username'");
		if(mysql_num_rows($db->result) == 1)
		{
			$db_password = mysql_result($db->result, 0, 1);
			if(md5($db_password . $this->salt) == $password)
			{
				$this->user_id = mysql_result($db->result, 0, 0);
				$this->username = $username;
				$this->ok = true;
				return true;
			}
		}			
		return false;
	}
 
	function logout()
	{
		$this->user_id = 0;
		$this->username = "Guest";
		$this->ok = false;
 
		$_SESSION['auth_username'] = "";
		$_SESSION['auth_password'] = "";
 
		setcookie("auth_username", "", time() - 3600, "/", $this->domain);
		setcookie("auth_password", "", time() - 3600, "/", $this->domain);
	}
 
}