Sweeper Installation Problem: Class 'PDO' not found


I'm trying to install Sweeper v. 0.3.1. The installer starts, but crashes (no output on the page) when trying to connect to the database. The log shows "PHP Fatal error: Class 'PDO' not found in .../sweeper/installer/steps/DBSetup.php on line 146". Line 146 of DBSetup.php is "$db = new PDO("mysql:host=$host;dbname=$database", $username, $password);"

I'm pretty sure the database name and credentials are correct. If I change the user, password or database to something else, I get a different error about not being able to access the database, but with them correct, I get an empty screen and the above error.

Help! Is there a way to do the installation manually if I can't get around this error?