Prevent user to use back button with warning or disable any insert

Question

I have a form in page1.php which directs to page2.php from where the data from the form in page1.php is inserted into database. after

Answer 1

Instead of giving link to a third page, redirect to the very same URI
this is quite handy method called POST/Redirect/GET:
here is a concise example of it:

<?php  
if ($_SERVER['REQUEST_METHOD']=='POST') {  

  $err = array();
  //performing all validations and raising corresponding errors
  if (empty($_POST['name'])) $err[] = "Username field is required";  
  if (empty($_POST['text'])) $err[] = "Comments field is required";  

  if (!$err) {  
    //if no errors - saving data and redirect
    header("Location: ".$_SERVER['PHP_SELF']);
    exit;
  }  else {
    // all field values should be escaped according to HTML standard
    foreach ($_POST as $key => $val) {
      $form[$key] = htmlspecialchars($val);
    }
  }
} else {
  $form['name'] = $form['comments'] = '';  
}
include 'form.tpl.php';
?>  

Here you can see another example, concise yet powerful: Separating Logic/Style in PHP properly
it's complete solution to display, add and edit database contents, exactly for admin purpose.

Answer 2

You should not display your message on page2.php.

Instead :

• page2.php should deal with the data
• when the data has been saved, page2.php should redirect to confirmation.php
• And it's only confirmation.php which would display the message.

For more informations, take a look at the Post/Redirect/Get pattern.

Edit after the comment : but note that, in any case, you will never be able to prevent the user from re-submitting a form, if he really wants to...

The only solution you'll have is, when a form is submitted, to check in your database if the currently submitted data already exists -- and if it does, refuse to insert it again.

Of course, if the suer changes even a single letter in his input, it won't be the same data anymore...