在使用Kevin Skoglund跟随Lynda.com啧啧超越基础时，会出现未定义的索引错误

Question

加载此页面时，我会继续收到以下通知：

  

（！）注意：未定义索引：清除

我很感激您在清除此通知时提供任何帮助。

<?php require_once("../../includes/initialize.php"); ?>
<?php if (!$session->is_logged_in()) { redirect_to("login.php"); } ?>
<?php

  $logfile = SITE_ROOT.DS.'logs'.DS.'log.txt';

  if($_GET['clear'] == 'true') {
        file_put_contents($logfile, '');
      // Add the first log entry
      log_action('Logs Cleared', "by User ID {$session->user_id}");
    // redirect to this same page so that the URL won't 
    // have "clear=true" anymore
    redirect_to('logfile.php');
  }
?>

<h2>Log File</h2>

<p><a href="logfile.php?clear=true">Clear log file</a><p>