Is user logged in variable with PHP

<?php if (Login::isAuthenticated()) { ?>
<a href="/sign-out/">Sign Out</a>
<?php } ?>

class Login
 {
    private static $_auth;

    public function __construct($username, $rawPassword) {

    	global $db;

    	require('edit/users/config.php');

    	$hashedPassword = sha1(SALT . $_POST['password']);

    	$query = 'SELECT firstname FROM users WHERE user = "' . $db->cleanString($username) . '" AND pass = "' . $db->cleanString($hashedPassword) . '" LIMIT 1';


    	$login = $db->query($query);

    	if ($login) {


    		$_SESSION['username'] = $username;
    		self::$_auth = true;



    		header('Location: ' . CONFIG_DIR_BASE);


    	} else {

    		ErrorHandler::addErrorToStack('Your username and/or password did not match one on our system. ');

    	}

    }



    public static function isAuthenticated() {



    	 return self::$_auth;


    }







 }