if(isset($_POST["login"]) || isset($_SESSION["username"]) && isset($_SESSION["password"]))
{
    $username=isset($_POST['username']) ? $_POST['username'] : $_SESSION["username"];
    $password=isset($_POST['password']) ? $_POST['password'] : $_SESSION["password"];
    if(empty($username)) $err[].="Username is empty";
    if(empty($password)) $err[].="Password is empty";
    if(!ctype_alnum($username.$password)) $err[].="Username and Password must contain only alphanumeric symbols!";
    if(!$err)
    {
        $sql = isset($_POST["login"])
             ? mysqli_prepare($con, "SELECT u_id FROM accounts WHERE u_username=? AND u_password=? LIMIT 1")
             : mysqli_prepare($con, "SELECT t_userid,t_ip FROM mem_login WHERE t_username=? AND t_password=? LIMIT 1");
        mysqli_stmt_bind_param($sql, "ss", $username, $password);
        mysqli_stmt_execute($sql);
        isset($_POST['login']) ? mysqli_stmt_bind_result($sql, $id) : mysqli_stmt_bind_result($sql, $id, $ip);
        mysqli_stmt_fetch($sql);
        mysqli_stmt_close($sql);
        if($id!=0 && isset($ip) ? $ip==$_SERVER['REMOTE_ADDR'] : TRUE)
        {
            define("LOGGED", TRUE, FALSE);
            $user["t_userid"]=$id;
            if(isset($_POST["login"]))
            {
                mysqli_query($con,"DELETE FROM mem_login WHERE t_userid=$id");
                $_SESSION["username"]=$username;
                $_SESSION["password"]=$password;
                $sql = mysqli_prepare($con, "INSERT INTO mem_login (t_userid,t_ip,t_username,t_password) VALUES (?,?,?,?)");
                mysqli_stmt_bind_param($sql, "isss", $id, $_SERVER['REMOTE_ADDR'], $username, $password);
                mysqli_stmt_execute($sql);
                mysqli_stmt_close($sql);
            }
        } else define("LOGGED", FALSE, FALSE);
    } else define("LOGGED", FALSE, FALSE);
} else define("LOGGED", FALSE, FALSE);