if($_SESSION['token'] == NULL){  $_SESSION['token'] = rand(1111,9999); }  if ( filter_input( INPUT_GET, 'r' ) == $_SESSION['token']){ $_SESSION['token'] = rand(1111,9999);  }  echo '<a href="/link?r='.$_SESSION['token'].'/"> Test</a>';   

   function gen_token() { $token = sprintf( '%04x%04x-%04x-%04x-%04x-%04x%04x%04x', mt_rand(0, 0xffff), mt_rand(0, 0xffff), mt_rand(0, 0xffff), mt_rand(0, 0x0fff) | 0x4000, mt_rand(0, 0x3fff) | 0x8000, mt_rand(0, 0xffff), mt_rand(0, 0xffff), mt_rand(0, 0xffff) );   return $token; }   echo gen_token(); // a30c8d89-f2aa-4af9-b1df-6b6443349706   

   <?php  $rand = mt_rand();  switch ($rand) {     case $rand:         echo 'Hello World!';         break; }