Install the php driver $ sudo pecl install mongodb
Create the extension file $ sudo nano /etc/php5/mods-available/mongodb.ini and write inside: extension=mongodb.so
Create a symbolic link for this file $ sudo ln -sv /etc/php5/mods-available/mongodb.ini /etc/php5/apache2/conf.d/20-mongodb.ini
Create an other symbolic link for this file $ sudo ln -sv /etc/php5/mods-available/mongodb.ini /etc/php5/cli/conf.d/20-mongodb.ini
Restart apache or the server used $ sudo service apache2 restart