How to prevent the "Confirm Form Resubmission" dialog?

This method works for me well and I think the simplest way to do this is to use this javascript code inside the reloaded page's HTML.

if ( window.history.replaceState ) {
  window.history.replaceState( null, null, window.location.href );
}

I was having this problem and added this JavaScript to the bottom of my page and it seemed to work. It seems much simpler a solution. Any drawbacks?

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

It seems you are looking for the Post/Redirect/Get pattern.

As another solution you may stop to use redirecting at all.

You may process and render the processing result at once with no POST confirmation alert. You should just manipulate the browser history object:

history.replaceState("", "", "/the/result/page")

See full or short answers

You could try using AJAX calls with jQuery. Like how youtube adds your comment without refreshing. This would remove the problem with refreshing overal.

You'd need to send the info necessary trough the ajax call.
I'll use the youtube comment as example.

$.ajax({
    type: 'POST',
    url: 'ajax/comment-on-video.php',
    data: {
        comment: $('#idOfInputField').val();
     },
    success: function(obj) {
        if(obj === 'true') {
            //Some code that recreates the inserted comment on the page.
        }
    }
});

You can now create the file comment-on-video.php and create something like this:

<?php
    session_start();

    if(isset($_POST['comment'])) {
        $comment = mysqli_real_escape_string($db, $_POST['comment']);
        //Given you are logged in and store the user id in the session.
        $user = $_SESSION['user_id'];

        $query = "INSERT INTO `comments` (`comment_text`, `user_id`) VALUES ($comment, $user);";
        $result = mysqli_query($db, $query);

        if($result) {
            echo true;
            exit();
        }
    }
    echo false;
    exit();
?>

I found an unorthodox way to accomplish this.

Just put the script page in an iframe. Doing so allows the page to be refreshed, seemingly even on older browsers without the "confirm form resubmission" message ever appearing.

Quick Answer

Use different methods to load the form and save/process form.

Example.

Login.php

Load login form at Login/index Validate login at Login/validate

On Success

Redirect the user to User/dashboard

On failure

Redirect the user to login/index