I sware i really looked everywhere out there about this, and if an answer already exists, i'm disapointed to learn that i'm blind…
Well. I'm quite interested in APC user-cache function because, unlike some php-level caching system, with APC the cached variables remain in memory, not on the hard drive. Which is good for speed.
Nevertheless, there are some cases of use i'm not sure concerning their effectiveness. For exemple, take a site which has its users logged in and has a dynamic menu as a sidebar (each user has its own specific menu).
Is it worth it to cache user-level variables (for instance the menu's content) ? Even though this menu is reloaded with each page and obviously creates an SQL request plus a PHP parsing ? Or does each user represent a too small amount of requests to make any difference (they won't load 50 pages a minute)?
Further more, we also have to take in account memory used. Will this saturate the server's memory ? I guess it heavily depends on how many users there is and how big are the menu's data… But i can't figure out how fast it will grow.
I know this makes a lot of questions, then thx in advance for your help :)
Unless your menus * # of users will result in gigabytes of session variables I'd stick with PHP sessions. They are easy and well tested.