Since ExpressionManager may be called hundreds of times per page, I'm using LimeExpessionManager.php as a Singleton  (so that I can cache results on a page per user).  Is that safe when there are multiple surveys being run against the same server, or should I be using $_SESSION to do caching?

(I'm used to Java where a Singleton would apply to the whole java virtual machine, so a Singleton' values would spill over to other sessions-  which is not what I want here).