After I have upgraded my system from Ubuntu 13.04 to 13.10 several problems have occurred with apache, mysql and php configurations.
I solved most of them but I can
I also had this problem with Ubuntu 14.04 and Nginx,but the tip for me was restart the FPM service, so I did:
Install the library
apt-get install php5-mcrypt
Find the path
updatedb && locate mcrypt.so
Set the path of mcrypt.so inside the mcrypt.ini file located in /etc/php5/mods-available/mcrypt.ini
extension=/usr/lib/php5/20121212/mcrypt.so
And then restart the FPM service
service php5-fpm restart