"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?