WHMCS v6 (currently in beta) ships with Smarty 3.

That case correlates to this forum thread http://forum.whmcs.com/showthread.php?100847-php-no-longer-working-since-Beta-4&highlight=beta+smarty

There are one or two other beta forum threads that discuss Smarty 3. If you wish to know more about v6 or join the beta program, please visit http://vsix.whmcs.com/