Use of undefined constant MCRYPT_BLOWFISH

No sooner do I speak do I find a solution, heh.

This worked for me: mCrypt not present after Ubuntu upgrade to 13.10

needed to go: sudo php5enmod mcrypt

even though it appeared to already be enabled, that fixed it.

When I try to enable the Express Paypal method, I came across this error.

Error: Use of undefined constant MCRYPT_BLOWFISH - assumed MCRYPT_BLOWFISH in /var/www/.../lib/Varien/Crypt/Mcrypt.php on line 56

Firstly, we can run this cmd to check the mcrypt is running or not:

php --ri mcrypt

If mcrypt is not enabled, we can use the following to enable it as others mentioned

sudo php5enmod mcrypt

As well, we had better check php5-mcrypt package is installed or not

dpkg -l | grep php5-mcrypt

While if you install fastcgi and php-fpm like us, we have to restart the php-fpm service, and apache2 service as well.

sudo service apache2 restart
sudo service php-fpm restart