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Preventing multiple PHP form submissions

When submitting a HTML form it can take several seconds before the form is successfully submitted and the response page shown. People can get inpatient and click the Submit button several times which can result in duplicate form submissions. Usually it's not really a problem, but in some cases you might want to prevent this from happening.

Below you will find two simple tricks for preventing duplicate submissions, you can use either of these or a combination of both.

» prevent multiple form submissions using Javascript

Using Javascript to block duplicate submissions is probably the easiest way. When someone submits the form we simply disable the Submit button and maybe change it's value to something more descriptive, like "Submitting, please wait..."

Try clicking this button for example. It will remain disabled until you reload this page:

The first step is to give your submit button a unique id, for example id="myButton":

<input type="submit" value="Submit" id="myButton" />

The second (and last) step is to give two Javascript commands to the <form> tag. The first one will tell the browser to disable the submit button after the form has been submitted and the second one will change the button text to give the user some idea about what's happening. This is the code to add to your form tag:

onsubmit="document.getElementById('myButton').disabled=true;
document.getElementById('myButton').value='Submitting, please wait...';"

Your form tag would then look something like:

<form action="contact.php" method="post"
onsubmit="document.getElementById('myButton').disabled=true;
document.getElementById('myButton').value='Submitting, please wait...';"
>

That's it. This trick should work in most modern browsers (IE 5+, FireFox, Opera, ...).

» prevent multiple form submissions using cookies

If you wish to avoid duplicate submissions for the entire browser session (or longer) you can consider using cookies. For example edit your form processing script to send a cookie to the browser after the form has been processed but before any HTML or redirection headers are printed. Placing this code after the mail() command should work in most cases:

setcookie('FormSubmitted', '1');

Then check for the cookie before processing. If it's there this visitor already submitted the form in active browser session. Add this code to the beginning of your form processing script:

if (isset($_COOKIE['FormSubmitted']))
{
    die('You may only submit this form once per session!');
}

That's it!

Here's how the final script from our php form tutorial would look like if modified to block duplicate submissions with cookies. Note some code was added at the top of the script and the cookie printing code after call to the mail() function:

<?php
/* Prevent duplicate submissions */
if (isset($_COOKIE['FormSubmitted']))
{
    show_error('You may only submit this form once per session!');
}

/* Set e-mail recipient */
$myemail  = "you@domain.com";

/* Check all form inputs using check_input function */
$yourname = check_input($_POST['yourname'], "Enter your name");
$subject  = check_input($_POST['subject'], "Write a subject");
$email    = check_input($_POST['email']);
$website  = check_input($_POST['website']);
$likeit   = check_input($_POST['likeit']);
$how_find = check_input($_POST['how']);
$comments = check_input($_POST['comments'], "Write your comments");

/* If e-mail is not valid show error message */
if (!preg_match("/([\w\-]+\@[\w\-]+\.[\w\-]+)/", $email))
{
    show_error("E-mail address not valid");
}

/* If URL is not valid set $website to empty */
if (!preg_match("/^(https?:\/\/+[\w\-]+\.[\w\-]+)/i", $website))
{
    $website = '';
}

/* Let's prepare the message for the e-mail */
$message = "Hello!

Your contact form has been submitted by:

Name: $yourname
E-mail: $email
URL: $website

Like the website? $likeit
How did he/she find it? $how_find

Comments:
$comments

End of message
";

/* Send the message using mail() function */
mail($myemail, $subject, $message);

/* Set a cookie to prevent duplicate submissions */
setcookie('FormSubmitted', '1');

/* Redirect visitor to the thank you page */
header('Location: thanks.htm');
exit();

/* Functions we used */
function check_input($data, $problem='')
{
    $data = trim($data);
    $data = stripslashes($data);
    $data = htmlspecialchars($data);
    if ($problem && strlen($data) == 0)
    {
        show_error($problem);
    }
    return $data;
}

function show_error($myError)
{
?>
    <html>
    <body>

    <b>Please correct the following error:</b><br />
    <?php echo $myError; ?>

    </body>
    </html>
<?php
exit();
}
?>

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