OneTimeURL

<?php

/* Enter the site address here,
 * must include the protocol used and the slash */
define("SITE_ADDR", "https://localhost/");

/* This is the key used for the HMAC
 * recommend to use `apg` (http://linux.die.net/man/1/apg)
 * or `openssl rand` to generate storng password */
define("MAC_KEY", "Put something random here");

/* This defined how long will the URL expire
 * Unit: Seconds
 * Default: 1 Hour */
define("EXPIRE_TIME", 60*60);

function GenerateURL($email) {
  $ttl = time() + EXPIRE_TIME;
  $email_hash = hash("md5", $email);  //can use urlencode to replace hashing
  $hmac = hash_hmac("sha256", $email_hash.$ttl, MAC_KEY);
  $query = "op=activate&id=$email_hash&expires=$ttl&signature=$hmac";

  return SITE_ADDR . "register.php?$query";
}

function VerifyURL($email_hash, $ttl, $hmac) {
  if($ttl < time()) { //Check is the link expired
    return false;
  } elseif($hmac != hash_hmac("sha256", $email_hash.$ttl, MAC_KEY)) { //Check the link signature
    return false;
  } else {
    return true;
  }
}