返回错误消息PHP

好吧，

我有表格登录到我的index.php和login.php我检查是否有错误。

我想将消息返回到Index.php，并通过$error_txt 显示消息

以下是我的代码：

Index.php:

<form action="login.php" method="post">
<input type="text" name="username" placeholder="Username" id="user-input" /><br />
<input type="password" name="password" placeholder="Password" id="user-input" /><br />
<input type="submit" name="login" value="Login" id="user-submit" class="btn btn-primary" style="color:white;"/><br />
<a href="register.php">Register</a>
<?php echo $error_txt; ?> <!-- --------- Here I Want show the error message -->
</form>

Login.php:

<?php    
if(isset($_POST['login']))
    {
        $username = mysql_escape_string($_POST['username']);
        $password = mysql_escape_string($_POST['password']);
        $errors = 0;
            try
            {
                if(strlen($username) == 0)
                {
                    throw new Exception ("The username is empty");
                }
                if(strlen($password) == 0)
                {
                    throw new Exception ("The password is empty");
                }
                include ("web/config.php");
                $usernameTrueQuery = mysql_query("SELECT * FROM `users` WHERE `Username` = '$username' AND `Password` = '$password' LIMIT 1;");
                $usernameTrue = mysql_num_rows($usernameTrueQuery);
                if($usernameTrue == 0)
                {
                    throw new Exception ("The username or password is incorrect");
                }
            } catch (Exception $loginError) {
                echo "<div class='alert alert-warning'>
              <strong>Error:</strong> ".$loginError->getMessage()."</div> 'n";
              $errors = 1;
              $error_txt = $loginError->getMessage();
            }
            if($errors == 0)
            {
                echo "<div class='alert alert-success'>
              <strong>Success</strong> Welcome {$username}!</div> 'n";
            }
    }
    return $error_txt;
    header("Location: /index.php");
?>

谢谢..：(