php Notice input data must be checked by xss_clean func include connec

<?php

/* 
    Notice: input data must be checked by ~ xss_clean func
*/

include("connect.php"); // DB settings & a lil bit sql-functions


if (!empty($_GET["get"]) == "logout"){
    logout();
}

// Get name of auth user
function get_auth_username(){
    if(!empty($_COOKIE['bbsessionhash'])){	
        DBSelect('forum');
        $sql = "select user.username, user.usergroupid from session, user
                where sessionhash='".$_COOKIE['bbsessionhash']."'
                and session.userid = user.userid";
        $user = Select2String($sql);
        $_SESSION['login'] = $user['username'];

        //echo $user["username"]; // debug
        return $user["username"]; // debug
    }
}

// Logout from vBulletin
function logout(){
    if(!empty($_COOKIE['bbsessionhash'])){
        DBSelect('forum');
        mysql_query("DELETE FROM session WHERE sessionhash="."'". $_COOKIE['bbsessionhash'] ."'");
        $_COOKIE = array();
    
        echo "Redirecting...";
        echo "<script language=\"JavaScript\">window.location = 'http://127.0.0.1/test/NetBeansProjects/PhpProject1/example.php';</script>";

    }
}