While your coding session, you encounter this error :

Use of undefined constant MCRYPT_BLOWFISH - assumed 'MCRYPT_BLOWFISH

You think, easy, a module should be missing on my php
So you install it :

sudo apt-get install php5-mcrypt

And restart apache.

But it still doesn’t work. In phpinfo() it seems the module is not installed.

In fact with the new version of PHP, you still need to activate the module

php5enmod -s apache2 mcrypt

The restart apache and the module should work.