I have a form on a php page that is submitted to the same page. I noticed that if I reload/refresh the page the form gets re-submitted. How do I code to avoid this in the most easy way?
One possibility is, to implement the post-redirect-get approach.
Simply said, a POST request will be never delivered to the browser. Instead you execute all necessary actions and store the information you need in the session, and then you make a redirect with code 303.
$page = 'show_result.php';
header('Location: '.$page, true, 303);
exit;
Doing it this way, the browser will show the "show_result.php" page (a GET request) instead of the page requested with POST. This is also the page that is added to the history, so refreshing and using the back button will never do another POST request. As a nice side effect you get rid of browser warnings about resending data, normally the user cannot decide what to do then anyway.
I think the biggest problem with this approach is, that you need a session to store error messages, that means you have to rely on cookies. If you do no redirect to display errors, the browser will show the warning about resending data.