6.3 How to prevent duplicate form submission

• 
• Certain web browsers - Read the 2^nd paragraphs in Post/Redirect/Get.

• Read the 3^rd paragraphs in Post/Redirect/Get.
• Read Redirect After Post.
• from http://blog.andreloker.de/post/2008/06/Post-Redirect-Get.aspx
• How to redirect?

  // After the POST at the server,
  header("Location: " . $_SERVER['REQUEST_URI']);  // Or other content as the result of the POST request
  // When 'refresh' is clicked, the browser will send GET with the URI to the server, and
  //   the server script of the URI sends HTML content or data
  

• With Ajax? No resubmission.

'SignIn':
    ...
    // include 'view_mainpage.php';  // Instead of this
    header('Location: view_mainpage.php');  // The browser will see 'view_mainpage.php' as a new page. The problem is that the session will be broken.

• Use of PRG pattern