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PHP: How to Fix Double Posting

When submitting a form, a user may click the submit button twice, they may reload the posted form, or hit back in their browser. Most of these will cause the form to be submitted twice and likely posted twice to the database.

Here’s one way to solve double posting, the process is:

  • generate a unique key and include it with the form HTML
  • when processing the form insert the unique key into the database table: transactions
    • the transactions table has a UNIQUE INDEX on the key field
    • if the insert fails, means there is a duplicate key, bail out of creating new one
  • batch job runs daily to clean up old transactions so the insert and index is kept tidy

Here’s a bit of code, writing it on-the-fly, so you may have to fix any typos or forgotten semi-colons. We use smarty templates and have custom functions built, so not as easy to share the exact code.

First you need to setup your transactions table:

CREATE TABLE transactions (
    transaction_key varchar(24),
    UNIQUE INDEX uki (transaction_key)
);

A simple PHP function to get a unique id, abstracted in case you want to change it later:

function get_transaction_key() {
    return uniqid(”, true);
}

The main function which checks if the key has been used before:

function check_transaction_key($key) {
    $return_value = db_insert(” INSERT INTO transactions (transaction_key) VALUES (’$key’) “);
    if ($return_value === false) { return false; }
    else { return true; }
}

Your PHP+HTML Template Script:

<form>
    <input type=”hidden” name=”transaction_key” value=”<?=get_transcaction_key()?>”>
    … extra form stuff…
</form>

Your PHP Form Process Script:

if (check_transaction_key($_REQUEST['transaction_key'])) {
    … fun form processing …
}

NOTE: The sample code above should not be considered best practices. Your PHP+HTML should be in a nice template and you should scrub all your submitted variables before doing anything with them.

Comments 8

  1. JL wrote:

    Or else, using javascript, you can hide the submit button when it’s been clicked once. It won’t help against spam (you probably already have a blocker anyway), but it’s easier to do.

    Posted 17 Jul 2007 at 2:49 am
  2. jasonjemse wrote:

    thank you for yur help :)

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