"Cannot modify header information" error

[mikek12004]

In top of my index I include funcs.php which is :

<?php
function GetCartId()
{
	//if user logged on use username as identification, else use coockie for anonymous user	
	if ($_SESSION['loggedon']==1)
	{
	$a=$_SESSION['username'];
	}			
	else
	{
	// This part generates an encrypted string and will set it as a cookie using set_cookie. This will also be used as the cookieId    //field in the cart table
		if(isset($_COOKIE["cartId"]))
		{
			return $_COOKIE["cartId"];
		}
		else
		{
			// There is no cookie set. We will set the cookie
			// and return the value of the users session ID
			
			session_start();
			setcookie("cartId", session_id(), time() + ((3600 * 24) * 30));
			$a=session_id();
			
			
		}
	}
	return $a;
}
function mssql_sec($a)
{
$a=str_replace("'","''",$a);
$a=str_replace(";","",$a);
return $a;
}
function mssql_sec2($a)
{
$a=str_replace("'","''",$a);
//$a=str_replace(";","",$a);
return $a;
}
?>

then later at the index I include basket.php which calls GetCartId() and the explorer shows the message
"Cannot modify header information - headers already sent by (output started at /var/www/html/generalm/demo/index.php:31) in /var/www/html/generalm/demo/nimda/funcs.php on line 22"
any help?
PS before including the funcs.php I have sesion_start()

[mikek12004]

The whole point is to identify a user so I can recognise him in future visits, even if I get the error the func works and a unique id (which is $a) is retrived which I can use it to display various info for him, any idea how can I disable the stupid error and why since it works this error is diplayed?