Prevent double posting with Post-Redirect-Get

[Donny Bahama]

I'm trying to implement measures that prevent double posting when a user clicks the "back" button after a form has been posted. I ran across this article on the "PRG (Post-Redirect-Get) method", but it's written at a very high level, discussing architecture and flow only; there's no how-to or example code.

I get the basic principle, more or less, but I have no idea how to implement it. (That's partly because I never use GET and I'm not completely sure how/when/why to do so.)

Has anyone used the PRG method for preventing double-posts? If so, can you please help me to understand it?

[JustinStudios]

I've always found the PRG method way to overbloated and uneccessary. Set a session variable and when your form is submitted successfully set that variable to sent. Tada, you just prevented yourself from getting the data 2 times AND you didn't have to do a whole lot of coding to prevent it.

[Donny Bahama]

But what if the user wants to post again during the same session?! This is a highly interactive site, so that will be common.

[grahame]

Donny, you can simply "drop" the contents of the cart that have been odered from $_SESSION when the order is placed and stay in the session otherwise. I agree that PRG is (for most uses) rather over the top.

I've been giving a PHP course over the last few days, and the issue of accepting an order into the system ONLY if it came through the right way (no doubling back, no short cuts!) came up. Important stuff for financial institutions. And I've ended up writing a blog item on it which I've just posted. The blog is here and the archived article will appear here if anyone is interested enough to want to get back to it later.