While setting up Drupal on a friend's Ubuntu server, I ran into the following error message:
PHP Fatal error: Undefined class constant 'MYSQL_ATTR_USE_BUFFERED_QUERY' in /var/www/mysite/includes/database/mysql/database.inc on line 42
This basically simply means that the PHP PDO extension has not been installed. It can be installed via something along the lines of:
sudo aptitude install php5-mysql
While you are at it, make sure that all the other PHP extensions that Drupal requires are also installed.