Symfony2 does not seem eager to identify the APC extension, despite the latter is being available as "php -m" demonstrates:
[PHP Modules] apc apcu bcmath bz2 ... ...
The logs show: FastCGI sent in stderr: "PHP message: PHP Fatal error: Uncaught exception 'RuntimeException' with message 'Unable to use ApcClassLoader as APC is not enabled.'
My PHP version is: 5.5.9. Any guesses would be appreciated.
UPDATE: phpinfo() somehow shows that APC is not enabled, however php -m includes it in.
So, the issue seems to be resolved. My box contains two php.ini files, one located at /cli/php.ini and another one at /fpm/php.ini.
I needed to uninstall and reinstall the APC package several times to see it actually working. Thanks all!