WHMCS is running a old, incomplete, and outdated version of Smarty 2.x I would like to see the smarty template engine upgraded to the full version of 3.x. Smarty 3 has better logic, performs better and has newer features that us developers could benefit from (for example: blocks and template inheritance). Also, smart 3.x has a backward compatible version available to support old 2.x templates that have a few features no longer supported. This could be included and the legacy system could be enabled with a config change.
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/