Fatal error: Call to undefined function mcrypt_encrypt()

What had worked for me with PHP version 5.2.8, was to open up php.ini and allow the php_mcrypt.dll extension by removing the ;, i.e. changing:

;extension=php_mcrypt.dll to extension=php_mcrypt.dll

For windows

;extension=php_mcrypt.dll to extension=php_mcrypt.dll 
 then restart your apache server

For Redhat

sudo yum install php55-mcrypt //if php5.5
sudo yum install php-mcrypt //if less than 5.4
sudo service httpd restart //if apache 2.4
sudo /etc/init.d/httpd restart //if apache 2.2 or less

For Ubuntu

sudo apt-get install php5-mcrypt
sudo service apache2 restart //if server not reloaded automatically 

Still not working?

sudo php5enmod mcrypt && sudo service apache2 restart

If you have recently updated to ubuntu 14.04 here is the fix to this problem:

$ sudo mv /etc/php5/conf.d/mcrypt.ini /etc/php5/mods-available/
$ sudo php5enmod mcrypt
$ sudo service apache2 restart