$ php-config --extension-dir
/usr/lib/php5/20121212
$ ll /usr/lib/php5/20121212 | grep redis
-rw-r--r-- 1 root root  255824 Mar  7  2014 redis.so
$ php -m | grep redis
$ sudo phpenmod redis
WARNING: Module redis ini file doesn't exist under /etc/php/5.6/mods-available
WARNING: Module redis ini file doesn't exist under /etc/php/5.6/mods-available

/usr/lib/php/20131226/