Detecting page reload after clicking back button in PHP - php

Question

I am looking for a way to detect when a page is reloaded after clicking the back button, so I can selectively ignore errors. Let me explain why:
Imagine a page that displays some data, identified by an id:
/show/some/data?id=5
Initially, the id exists and the page is displayed. If the id does not exist, an error message is shown and an error is logged, because that should not happen and would indicate a broken link in the application.
Now imagine the user looks at the data, decides to delete it, and after deleting, uses the back button to go to the same page again (BTW this is not a hypothetical, I have seen this in my application). Now, the id is no longer valid, triggering an error. If I could distinguish between the initial page load and the second page load after using the back button, I could be more lenient in my error handling. My reasoning being that a broken (forward) link in the application is a fatal error. A broken link after clicking back (when there was none initially) is very likely caused by some action the user took in the meantime. In that case I would much rather redirect the user to an overview page than show an application error.
I believe one way to solve this would be to attach some randomly generated id to the url:
/show/some/data?id=5&rnd=<?php print rand(); ?>
and log the random ids that have been used in the session. If an id repeats itself, I can assume that the page has been reloaded, and selectively ignore some errors.
However, that would require altering many links in the application. I could also use javascript/jQuery to alter all links on the page to the same effect:
$("a").attr('href', function (i, h) {
return h + (h.indexOf('?') != -1 ? "&" : "?") + "rnd=" + Math.random ();
});
That's slightly less invasive, but still rather heavy handed for solving such a seemingly trivial problem. I wonder if there is or can be a self-contained way to do this. Is there any way for the backend application to distinguish if a page is loaded by forward clicking on a link or reloaded after clicking back? Or is there a more elegant solution with javascript?

Answer 1

There's no perfect solution for this since you're trying to (sort of) change a browser's behaviour. The simplest idea I can think of is setting a session variable with a list of deleted pages, and pushing the page IDs into it on the delete page.
Then, you need to update your error handler to additionally check for the presence of the requested page ID in the deleted pages list. That way you won't have to mess with URLs at all.
Here's an example flow:
user requests page id 1
app checks the db, it doesn't exist
app checks the session, it's not in the deleted pages list
error message shown, error logged on the server
user requests page id 2
app checks the db, it exists, show the page to the user
user clicks the delete button
ID of the page is added to the session list of deleted page and the page is deleted
user goes back (or types the url manually, this is a bonus feature)
app checks the db, id 2 doesn't exist
app checks the session, id 2 IS in the deleted pages list
error message shown, no errors logged
You can also take this one step further and track all the visited pages, so you'll know that if something was there a few minutes ago, and now it isn't, the user must have deleted it. This would allow you to handle more such events if needed.