http://source.keyboard-monkeys.org/wiki/communityid/How_to_install_Community-ID_under_Linux

sudo apt-get install php-pear php5-dev libmysqlclient15-dev
sudo pecl install pdo
sudo pecl install pdo_mysql
sudo gedit /etc/php5/apache2/php.ini

Add these:

extension=pdo.so
extension=pdo_mysql.so