You have not installed MongoDB PHP driver please see this link http://www.php.net/manual/en/mongo.installation.php
Update sources
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
echo 'deb http://downloads-distro.mongodb.org/repo/ubuntu-upstart dist 10gen' | sudo tee /etc/apt/sources.list.d/mongodb.list
sudo apt-get update
Install MongoDB PHP Driver
sudo apt-get install php5-dev php5-cli php-pear -y
sudo pecl install mongo
Open your php.ini file and add to it:
extension=mongo.so
Restart apache
sudo /etc/init.d/apache2 restart
Other helping info:
this should help to find your php.ini file:
php -i | grep 'Configuration File'
On Ubuntu it shows this:
Configuration File (php.ini) Path => /etc/php5/cli
Loaded Configuration File => /etc/php5/cli/php.ini
take a note, that you run this command from cli (command line) so for your true php.ini go to folder apache2 instead of cli 🙂