While working on a Magento-based store, I cam across this error:
Fatal error: Call to undefined function mcrypt_module_open() in /path/to/Magento/site/lib/Varien/Crypt/Mcrypt.php on line 63
I had never encounter this error before, so I did a quick search, and found out that the problem was a missing module on php. The missing module was mCrypt.
phpinfo() confirmed that the module was not installed. Once I knew that, it was just a matter of installing that module, and restarting the apache server.
To install mCrypt on Ubuntu is very simple:
sudo apt-get install php5-mcrypt
And to restart apache:
sudo /etc/init.d/apache2 restart
This solves the problem.