apc_cache for php 5.4.x

db.php recommends apc_cache module's user variable caching functions. If apcu is available than db.php uses it to store cache values by default. You can also override default caching engine but it is subject for further reading. A link for apc_cache installation instructions is here http://php.net/manual/en/apc.installation.php. On windows you just need to download proper php_apc.dll and enable it in php.ini by uncommenting line extension=php_apc.dll

In linux it is a bit difficult but usually you will and up with this commands:

wget http://pecl.php.net/get/APC-3.1.9.tgz
tar -xvf APC-3.1.9.tgz
cd APC-3.1.9
./configure -enable-apc -enable-apc-mmap -with-apxs2=/usr/sbin/apxs
make test
make install