PHP, PDO, sqlite "could not find driver"

I'm at a loss. In my PHP scripts, I'm using PDO to create/access a sqlite3 database. However, on new PDO( ... ), an exception is throw with the reason: could not find driver

Can anyone tell me how I can tweak PHP to get PDO/sqlite3 working?
Do I need to download something? I don't care if I have to build a module, but I'll be very, very upset [with Apple] if the solution requires me to rebuild PHP.