How to prevent form resubmission when page is refreshed (F5 / CTRL+R)

I would also like to point out that you can use a javascript approach, window.history.replaceState to prevent a resubmit on refresh and back button.

<script>    if ( window.history.replaceState ) {        window.history.replaceState( null, null, window.location.href );    }</script>

Proof of concept here: https://dtbaker.net/files/prevent-post-resubmit.php

I would still recommend a Post/Redirect/Get approach, but this is a novel JS solution.

Use the Post/Redirect/Get pattern. http://en.wikipedia.org/wiki/Post/Redirect/Get

With my website, I will store a message in a cookie or session, redirect after the post, read the cookie/session, and then clear the value of that session or cookie variable.

You can prevent form resubmission via a session variable.

First you have to set rand() in a textbox and $_SESSION['rand'] on the form page:

<form action="" method="post">  <?php   $rand=rand();   $_SESSION['rand']=$rand;  ?> <input type="hidden" value="<?php echo $rand; ?>" name="randcheck" />   Your Form's Other Field  <input type="submit" name="submitbtn" value="submit" /></form>

After that check $_SESSION['rand'] with textbox $_POST['randcheck'] valuelike this:

if(isset($_POST['submitbtn']) && $_POST['randcheck']==$_SESSION['rand']){    // Your code here}

Make sure you start the session on every file you are using it with session_start()