newbie to www-athenticate via sessions

[kosmidd]

I have these two functions in a class:

class admin{
	function SessionClear() { 
		$_SESSION=array();
		unset($_COOKIE[session_name()]);
		session_destroy();
		$time=time();
		header("Location: http://".$_SERVER["SERVER_NAME"].$_SERVER["PHP_SELF"]."?ref=1&cache_defeat=$time");
		exit;
	} 


function Authenticate() {
define("HTTP_AUTH_REALM", "123");
session_start();
	if(!isset($_SESSION["uid"])) {
		if(!isset($_COOKIE['login_attempts'])){
			unset($_SERVER['PHP_AUTH_USER']);
			setcookie('login_attempts', 1,time()+300);
			echo$_COOKIE['login_attempts']."121212";
		}
		if(!isset($_SERVER['PHP_AUTH_USER'])){
			header("WWW-Authenticate: Basic realm=\"".HTTP_AUTH_REALM."\"");
			header("HTTP/1.0 401 Unauthorized");
			echo("This is for authorized users only.");
			exit;
		}
		if($_SERVER['PHP_AUTH_USER']!='u' || $_SERVER['PHP_AUTH_PW']!='p') {
			echo("This is for authorized users only.");
			exit;
		}
	}
}
}

It doesn't seem to work correctly.
I call it like this
In every protected page:

$aa=new Admin();
$aa->Authenticate();

It asks the fisrt time for password but it never refresh the internal time.When 300 secs pass it requires login again.
When a user hits logout.

$aa->SessionClear();

It nevers logout.
How can I make this work with session control?
It seems to work only with cookies and again not correct