APC is gone, no longer maintained, now there are alternatives.
For opcode cache, PHP 5.5 (5.6 shipps with jessie) the opcode cacher ( OPcache ) module is installed and enabled by default, as for key cache (Persistent across pages), we have an alternative called APCu (Just the user key value cache), once installed, the apc_ functions return to PHP so it is a drop in replacement, no no program modifications needed.
To install APCu, you run the following
1- Install the tools
apt-get install apache2-threaded-dev php5-dev php-pear make
2- Before you get APCu-4.0.7, you should check what the latest version is !
pecl install channel://pecl.php.net/APCu-4.0.7
Now you are done, all you need to do is add
extension=apcu.so
to the php config file, in my case, what i do on debian jessie is add it in a file here
/etc/php5/apache2/conf.d/apcu.ini
There you have it, you are back on track.