That's funny, because as soon as Salathe posted that I was going to throw in a mention that the filter_* functions weren't added until 5.2.0. I've ran into this problem twice in the past little while with a few public domain projects I've released. First time was using htmlentities double_encode argument, and the second was with filter_var.

Upgrade PHP and you'll be good to go. `aptitude update | aptitude safe-upgrade` and if you're not on Debian, upgrade to that while you're at it. :D