Rollback the transaction when user abort the connection

Question

First of all, sorry for my english, I wrote this with some help of Google Translate.

I'm trying to make an application like Google Wave with PHP and Ajax. I have a textarea that when the user input something, the javascript on the page detected with oninput and send the contents of the textarea to the server and the server stores the contents into the database.

What I'm doing is that every time when i send the content by XHR, there is XHR.abort() that always interrupts the previous XHR request. The data that is in the database are fine, however, sometimes it is stored a previous version.

I know it happens because PHP has not stopped the execution even though the client has made an abort and sometimes the previous request has taken more time that the last request and completed after the last request, so I read the manual of functions of "ignore_user_abort" and "connection_aborted", but the problem persist.

I created this script to simulate the situation and I hoped when I aborted the connection (press 'stop', close the tab/window), there are not any new data on the database, but after 5 seconds, there still I have new data, so I need help to rollback the transaction when user abort the connection.

Here is the script to simulate (PDO_DSN, PDO_USER, PDO_PASS are defined):

<?php
ignore_user_abort(true);

ob_start('ob_gzhandler');

$PDO = new PDO(PDO_DSN, PDO_USER, PDO_PASS, array(PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8'));

$PDO->beginTransaction();
$query = $PDO->query('INSERT INTO `table` (`content`) VALUES (' . $PDO->quote('test') . ')');
sleep(5);
echo ' ';
ob_flush();
flush();
if (connection_aborted()) {
  $PDO->rollBack();
  exit;
}
$PDO->commit();

ob_end_flush();

Answer 1

How about having the browser send back a timestamp or a running number that you also store in the database. and your update can check so that it only writes if the new timestamp is newer.