很好地缓存了很多变量？

We have an Android App with large number of users (about 300K) with lot of requests and calculations using a PHP web-service.

we have a function named getUserRank that takes user_id & timePeriod to calculate the user rank according to 3 tables (using huge joins) and we use it in our app a lot for every single user.

/*
 * get user rank of the user according to the specified period
 * periods: 0-> all  1-> daily  2-> weekly 3-> monthly
 */
public function getUserRank($user_id, $timePeriod){
// piece of code
}

I use Xcache and mod_php on my server. Is this a good job to store all of the user ranks for few hours before requesting from database in such a way (to send them by json to the App)?

$rank = xcache_get($user_id.$timePeriod); // generate cache id using $user_id.$timePeriod
if($rank == null){
// calculate and populate the cache variable
}