Session variables are tied to user sessions (like name suggests), so they cannot be used to persist application state (e.g. cached content, compiled templates, metrics, etc.)
I see downvotes on my GP post, I'm simply stating the fact: if you need state for a building a powerful webapp then obviously there's better runtimes that PHP+Apache, I don't see how one could disagree with that statement, apart from not having sufficient experience in that matter.
There's plenty of good reasons to counterbalance that fact thought, PHP has a very good ecosystem, multiple mature/powerful web frameworks, and cheap/abundant workforce. But it lacks a state holding runtime and for the most demanding applications it's a sufficiently big technical hassle to encourage seeking other runtime and/or language.