Learn how to use PHP sessions to prevent duplicate form submission or re-submission.
HTTP is a stateless protocol. That is, a HTTP request to a website has no information or record of the preceding requests. This is where sessions come into picture. Sessions is a mechanism to preserve data across subsequent accesses. Using sessions, a website is able to maintain state, that is, remember which requests they received previously. Without sessions, websites would have not have progressed beyond simple static HTML pages.