Problem: Magento Shopping cart displays blank page when items are clearly in the cart. Server Error log will display the following message: Call to undefined function mcrypt_module_open()

Solution: PHP mcrypt() Module is not installed, install mcrypt to resolve this issue.

To install mCrypt for PHP5 on your Ubuntu Linux system:

sudo apt-get install php5-mcrypt

Then restart Apache with:

sudo /etc/init.d/apache2 restart

Additionally, this problem can present itself in the admin panel as blank pages that don’t resolve, and it also throws a function error on every login.