I'm with paulkd on this one... using a database table is pretty simple and sidesteps the (purposeful) isolation between sessions PHP guarantees.
When I implemented it, it was as an after-though, and then only because it was so simple to do (I already have a table that keeps tabs on who's signed in, when they signed in, from where, and things like that... so, in my case (where only the total is wanted), it's a simple conditional COUNT, but it would be simple enough to change the COUNT to a SELECT).