PDOException “could not find driver”

I have just installed Debian Lenny with Apache, MySQL, and PHP and I am receiving a PDOException `could not find driver`. This is the specific line of code it is referring to: `$dbh = new PDO('mysql:host=' . DB_HOST . ';dbname=' . DB_NAME, DB_USER, DB_PASS)` `DB_HOST`, `DB_NAME`, `DB_USER`, and `DB_PASS` are constants that I have defined. It works fine on the production server (and on my previous Ubuntu Server setup). Is this something to do with my PHP installation? Searching the internet has not helped, all I get is experts-exchange and examples, but no solutions.