How to install APC under PHP 5.6?
APC is installed
apt-get install php-pear php5-dev make libpcre3-dev
pecl install apc
# locate apc.so
/usr/lib/php5/20100525/apc.so
APC is added to php.ini
extension=apc.so
But APC is not mentioned in output from phpinfo()
Fatal error: Call to undefined function apc_fetch()
APC is (more or less) a deprecated package (the last release, 3.1.14, was unstable and had to be rolled back). It has been replaced by the core package opcache.
I'm not sure about Debian flavors (all my searches return the PECL library while opcache is native to 5.6) but in CentOS you have to install the php-opcache package, which contains the opcahce.so file.