PHP should already have PDO extension enabled. What you probably need is the mysql driver for it.

# test if php5-mysql package is installed
dpkg-query --status php5-mysql | grep Status

# install the package if the above command indicates that it is not installed
sudo apt-get install php5-mysql

[Edit: oops, @ryanpq already answered]

You can do this with:

sudo apt-get update
sudo apt-get install pdo-mysql php5-mysql

If after this you are unable to use the extension you can check your /etc/php5/apache2/php.ini and make sure it includes:

extension=pdo.so
extension=pdo_mysql.so