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

I have a simple form that submits text to my SQL table. The problem is that after the user submits the text, they can refresh the page and the data gets submitted again without filling the form again. I could redirect the user to another page after the text is submitted, but I want users to stay on the same page.

I remember reading something about giving each user a unique session id and comparing it with another value which solved the problem I am having but I forgot where it is.

Basically, you need to redirect out of that page but it still can make a problem while your internet slow (Redirect header from serverside)

Example of basic scenario :

Click on submit button twice

Way to solve

• Client side

  • Disable submit button once client click on it
  • If you using Jquery : Jquery.one
  • PRG Pattern

• Server side

  • Using differentiate based hashing timestamp / timestamp when request was sent.
  • Userequest tokens. When the main loads up assign a temporary request tocken which if repeated is ignored.

How to prevent php form resubmission without redirect. If you are using $_SESSION (after session_start) and a $_POST form, you can do something like this:

if ( !empty($_SESSION['act']) && !empty($_POST['act']) && $_POST['act'] == $_SESSION['act'] ) {
  // do your stuff, save data into database, etc
}

In your html form put this:

<input type="hidden" id="act" name="act" value="<?php echo ( empty($_POST['act']) || $_POST['act']==2 )? 1 : 2; ?>">
<?php
if ( $_POST['act'] == $_SESSION['act'] ){
    if ( empty( $_SESSION['act'] ) || $_SESSION['act'] == 2 ){
        $_SESSION['act'] = 1;
    } else {
        $_SESSION['act'] = 2;
    }
}
?>

So, every time when the form is submitted, a new act is generated, stored in session and compared with the post act.

Ps: if you are using an Get form, you can easily change all POST with GET and it works too.

A refined version of Moob's post. Create a hash of the POST, save it as a session cookie, and compare hashes every session.

// Optionally Disable browser caching on "Back"
header( 'Cache-Control: no-store, no-cache, must-revalidate' );
header( 'Expires: Sun, 1 Jan 2000 12:00:00 GMT' );
header( 'Last-Modified: ' . gmdate('D, d M Y H:i:s') . 'GMT' );

$post_hash = md5( json_encode( $_POST ) );

if( session_start() )
{
    $post_resubmitted = isset( $_SESSION[ 'post_hash' ] ) && $_SESSION[ 'post_hash' ] == $post_hash;
    $_SESSION[ 'post_hash' ] = $post_hash;
    session_write_close();
}
else
{
    $post_resubmitted = false;
}

if ( $post_resubmitted ) {
  // POST was resubmitted
}
else
{
  // POST was submitted normally
}

This form.php sample shows how to use PRG correct (when form is valid or not).

• It redirects to the same page, only when form is valid and action was performed.
• Redirection protects form from being resubmitted on page refresh.
• It uses session to not loose success messages you want to show when form is valid.
• There are two buttons for testing: "Valid submit", "Invalid submit". Try both and refresh page after that.

<?php
session_start();

function doSelfRedirect()
{
  header('Location:'.$_SERVER['PHP_SELF']);
  exit;
}

function setFlashMessage($msg)
{
  $_SESSION['message'] = $msg;
}

function getFlashMessage()
{
  if (!empty($_SESSION['message'])) {
    $msg = $_SESSION['message'];
    unset($_SESSION['message']);
  } else {
    $msg = null;
  }

  return $msg;
}

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
  // Validation primitive example.
  if (empty($_POST['valid'])) {
    $formIsValid = false;
    setFlashMessage('Invalid form submit');
  } else {
    $formIsValid = true;
  }

  if ($formIsValid) {
    // Perform any actions here.
    // ...

    // Cool!
    setFlashMessage('Form is valid. Action performed.');

    // Prevent form resubmission.
    doSelfRedirect();
  }
}
?>
<h1>Hello form</h1>

<?php if ($msg = getFlashMessage()): ?>
  <div><?= $msg ?></div>
<?php endif; ?>

<form method="post">
  <input type="text" name="foo" value="bar"><br><br>
  <button type="submit" name="invalid" value="0">Invalid submit</button>
  <button type="submit" name="valid" value="1">Valid submit</button>
</form>

Just redirect it to the same page after making the use of form data, and it works. I have tried it.

header('location:yourpage.php');

if (($_SERVER['REQUEST_METHOD'] == 'POST') and (isset($_SESSION['uniq']))){
    if($everything_fine){
        unset($_SESSION['uniq']);
    }
}
else{
    $_SESSION['uniq'] = uniqid();
}

When the form is processed, you redirect to another page:

... process complete....
header('Location: thankyou.php');

you can also redirect to the same page.

if you are doing something like comments and you want the user to stay on the same page, you can use Ajax to handle the form submission

As others have said, it is not possible to out of using post/redirect/get. But at the same time it is quite easy to do what you want to do server side.

In your POST page you simply validate the user input but do not act on it, instead you copy it into a SESSION array. You then redirect back to the main submission page again. Your main submission page starts by checking to see if the SESSION array that you are using exists, and if so copy it into a local array and unset it. From there you can act on it.

This way you only do all your main work once, achieving what you want to do.

I searched for solution to prevent resubmission in a huge project afterwards. The code highly works with $_GET and $_POST and I can't change the form elements behaviour without the risk of unforeseen bugs. So, here is my code:

<!-- language: lang-php -->
<?php

// Very top of your code:

// Start session:
session_start();

// If Post Form Data send and no File Upload
if ( empty( $_FILES ) && ! empty( $_POST ) ) {
    // Store Post Form Data in Session Variable
    $_SESSION["POST"] = $_POST;
    // Reload Page if there were no outputs
    if ( ! headers_sent() ) {
        // Build URL to reload with GET Parameters
        // Change https to http if your site has no ssl
        $location = "https://" . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
        // Reload Page
        header( "location: " . $location, true, 303 );
        // Stop any further progress
        die();
    }
}

// Rebuilt POST Form Data from Session Variable
if ( isset( $_SESSION["POST"] ) ) {
    $_POST = $_SESSION["POST"];
    // Tell PHP that POST is sent
    $_SERVER['REQUEST_METHOD'] = 'POST';
}

// Your code:
?><html>
    <head>
        <title>GET/POST Resubmit</title>
    </head>
    <body>

    <h1>Forms:</h1>
    <h2>GET Form:</h2>
    <form action="index.php" method="get">
        <input type="text" id="text_get" value="test text get" name="text_get"/>
        <input type="submit" value="submit">
    </form>
    <h2>POST Form:</h2>
    <form action="index.php" method="post">
        <input type="text" id="text_post" value="test text post" name="text_post"/>
        <input type="submit" value="submit">
    </form>
    <h2>POST Form with GET action:</h2>
    <form action="index.php?text_get2=getwithpost" method="post">
        <input type="text" id="text_post2" value="test text get post" name="text_post2"/>
        <input type="submit" value="submit">
    </form>
    <h2>File Upload Form:</h2>
    <form action="index.php" method="post" enctype="multipart/form-data">
        <input type="file" id="file" name="file">
        <input type="submit" value="submit">
    </form>

    <h1>Results:</h1>
    <h2>GET Form Result:</h2>
    <p>text_get: <?php echo $_GET["text_get"]; ?></p>
    <h2>POST Form Result:</h2>
    <p>text_post: <?php echo $_POST["text_post"]; ?></p>
    <h2>POST Form with GET Result:</h2>
    <p>text_get2: <?php echo $_GET["text_get2"]; ?></p>
    <p>text_post2: <?php echo $_POST["text_post2"]; ?></p>
    <h2>File Upload:</h2>
    <p>file:
    <pre><?php if ( ! empty( $_FILES ) ) {
            echo print_r( $_FILES, true );
        } ?></pre>
    </p>
    <p></p>
    </body>
    </html><?php
// Very Bottom of your code:
// Kill Post Form Data Session Variable, so User can reload the Page without sending post data twice
unset( $_SESSION["POST"] );

It only works to avoid the resubmit of $_POST, not $_GET. But this is the behaviour I need. The resubmit issue doesn't work with file uploads!

I found next workaround. You may escape the redirection after processing POST request by manipulating history object.

So you have the HTML form:

<form method=POST action='/process.php'>
 <input type=submit value=OK>
</form>

When you process this form on your server you instead of redirecting user to /the/result/page by setting up the Location header like this:

$cat process.php
<?php 
     process POST data here
     ... 
     header('Location: /the/result/page');
     exit();
?>

After processing POSTed data you render small <script> and the result /the/result/page

<?php 
     process POST data here
     render the <script>         // see below
     render `/the/result/page`   // OK
?>

The <script> you should render:

<script>
    window.onload = function() {
        history.replaceState("", "", "/the/result/page");
    }
</script>

The result is:

as you can see the form data is POSTed to process.php script.
This script process POSTed data and rendering /the/result/page at once with:

1. no redirection
2. no rePOST data when you refresh page (F5)
3. no rePOST when you navigate to previous/next page through the browser history

UPD

As another solution I ask feature request the Mozilla FireFox team to allow users to setup NextPage header which will work like Location header and make post/redirect/get pattern obsolete.

In short. When server process form POST data successfully it:

1. Setup NextPage header instead of Location
2. Render the result of processing POST form data as it would render for GET request in post/redirect/get pattern

The browser in turn when see the NextPage header:

1. Adjust window.location with NextPage value
2. When user refresh the page the browser will negotiate GET request to NextPage instead of rePOST form data

I think this would be excelent if implemented, would not? =)

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'] value like 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()

What Works For Me is :

if ( !refreshed()) {
   //Your Submit Here
        if (isset( $_GET['refresh'])) {
            setcookie("refresh",$_GET['refresh'], time() + (86400 * 5), "/");
        }

    }    
}


function refreshed()
{
    if (isset($_GET['refresh'])) {
        $token = $_GET['refresh'];
        if (isset($_COOKIE['refresh'])) {
            if ($_COOKIE['refresh'] != $token) {
                return false;
            } else {
                return true;
            }
        } else {
            return false;
        }
    } else {
        return false;
    }
}  


function createToken($length) {
    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $charactersLength = strlen($characters);
    $randomString = '';
    for ($i = 0; $i < $length; $i++) {
        $randomString .= $characters[rand(0, $charactersLength - 1)];
    }
    return $randomString;
}

?>

And in your Form

 <form  action="?refresh=<?php echo createToken(3)?>">



 </form>

1. Use header and redirect the page.

   header("Location:your_page.php"); You can redirect to same page or different page.

2. Unset $_POST after inserting it to Database.

   unset($_POST);

This method works for me well, and I think this is the simplest one to do this job.

The general idea is to redirect the user to some other pages after the form submission, which would stop the form resubmission on page refresh. Still, if you need to hold the user on the same page after the form is submitted, you can do it in multiple ways, but here I am describing the javascript method.

Javascript Method

This method is quite easy and blocks the pop up asking for form resubmission on refresh once the form is submitted. Just place this line of javascript code at the footer of your file and see the magic.

<script>_x000D_
if ( window.history.replaceState ) {_x000D_
  window.history.replaceState( null, null, window.location.href );_x000D_
}_x000D_
</script>

I use this javascript line to block the pop up asking for form resubmission on refresh once the form is submitted.

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

Just place this line at the footer of your file and see the magic

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.

You should really use a Post Redirect Get pattern for handling this but if you've somehow ended up in a position where PRG isn't viable (e.g. the form itself is in an include, preventing redirects) you can hash some of the request parameters to make a string based on the content and then check that you haven't sent it already.

//create digest of the form submission:

    $messageIdent = md5($_POST['name'] . $_POST['email'] . $_POST['phone'] . $_POST['comment']);

//and check it against the stored value:

    $sessionMessageIdent = isset($_SESSION['messageIdent'])?$_SESSION['messageIdent']:'';

    if($messageIdent!=$sessionMessageIdent){//if its different:          
        //save the session var:
            $_SESSION['messageIdent'] = $messageIdent;
        //and...
            do_your_thang();
    } else {
        //you've sent this already!
    }

Using the Post/Redirect/Get pattern from Keverw answer is a good idea. However, you are not able to stay on your page (and I think this was what you were asking for?) In addition, it may sometimes fail:

If a web user refreshes before the initial submission has completed because of server lag, resulting in a duplicate HTTP POST request in certain user agents.

Another option would be to store in a session if text should be written to your SQL database like this:

if($_SERVER['REQUEST_METHOD'] != 'POST')
{
  $_SESSION['writeSQL'] = true;
}
else
{
  if(isset($_SESSION['writeSQL']) && $_SESSION['writeSQL'])
  {
    $_SESSION['writeSQL'] = false;

    /* save $_POST values into SQL */
  }
}

Why not just use the $_POST['submit'] variable as a logical statement in order to save whatever is in the form. You can always redirect to the same page (In case they refresh, and when they hit go back in the browser, the submit post variable wouldn't be set anymore. Just make sure your submit button has a name and id of submit.

Javascript

This method is quite easy and blocks the pop up asking for form resubmission on refresh once the form is submitted. Just place this line of javascript code at the footer of your file and see the magic.

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

After inserting it to database, call unset() method to clear the data.

unset($_POST);

To prevent refresh data insertion, do a page redirection to same page or different page after record insert.

header('Location:'.$_SERVER['PHP_SELF']);

A pretty surefire way is to implement a unique ID into the post and cache it in the

<input type='hidden' name='post_id' value='".createPassword(64)."'>

Then in your code do this:

if( ($_SESSION['post_id'] != $_POST['post_id']) )
{
    $_SESSION['post_id'] = $_POST['post_id'];
    //do post stuff
} else {
    //normal display
}

function createPassword($length)
{
    $chars = "abcdefghijkmnopqrstuvwxyz023456789";
    srand((double)microtime()*1000000);
    $i = 0;
    $pass = '' ;

    while ($i <= ($length - 1)) {
        $num = rand() % 33;
        $tmp = substr($chars, $num, 1);
        $pass = $pass . $tmp;
        $i++;
    }
    return $pass;
}