How to expire the activation link in PHP?

Question

I have a php script that sends an activation link via email to the users so they can activate their account. The link is like this: mysite.com/activation.phpid?id=20

Answer 1

Make the link like this:

$time = time();
$hash = md5($id . $time . "somerandomsalt"); // check this again in activation.php
$link = "activation.php?id=" . $id . "&hash=" . $hash . "&time=" . $time;

Then in activation.php you check if the hash matches. Oh, and check the time of course :P

You could obfuscate it a bit to hide the id, hash and time query parameters, but this is the basics.