After installing lamp-server or the individual php5-mcrypt package, you may come across the following error (e.g. in phpMyAdmin):
The mcrypt extension is missing. Please check your PHP configuration.
The problem is simply because the module is not enabled by default after installation. To enable, execute:
sudo php5enmod mcrypt
and then restart the Apache server using:
sudo /etc/init.d/apache2 restart
sudo service apache2 restart