Preventing Repeat Form Submission Using PHP Sessions

We've all seen those messages on some websites warning not to click a button more than once or negative consequences, like paying a bill twice, may result. Sometimes we can cause these problems by hitting the back or refresh buttons. In this article I will explain a methodology whereby a site can ensure each form is submitted only once, thereby demonstrating that such warnings are unnecessary and, depending on the nature of the problems caused, worth repairing immediately. Let's begin by taking a look at the process we are studying: Form Submission. As pedantic as it may seem, it will be worthwhile to detail each of the steps in this process:

Since the item which changes does not depend on the form being submitted (e.g. it doesn't matter if it's a newsletter registration form, customer signup form, payment form, etc.), the item is not something which already exists and therefore must be created, so let's create a form variable called submissionId and assume it has the 3 properties mentioned above. So far, so good -- or so it appears! The third "property" is that "we can tell [it] changed", but "changed" is not a property of a variable, so we need to look at this more closely. In order to tell something changed, we must have a reference point, an answer to the question "changed from what?" This is where a session variable will come into play. If we define a session variable, say $_SESSION['nextValidSubmission'] and treat it as a reference point, we will have all of the tools necessary to protect our visitors. The idea will be to keep the session variable updated with the last submissionId sent out and change the submissionId each time it is sent out to the user. Then, if they try to resubmit the data, they will be submitting an old submissionId which doesn't match nextValidSubmission and we will know not to re-process this data. Let's look at this in terms of the processes:

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See here for more detailed information, an example using PHP, and an alternate method which doesn't require sessions.