joebeeson/lithium — comment on commit 1bb901c8047fd673166daba2fb12f1afea139f3f
File:  line 0
By joebeeson:

According to the `filter_var` documentation the function _should_ return `FALSE` for an empty string value, which indicates that the value is a boolean, it's just "off" -- unfortunately PHP does not function that way hence the check for `empty` which should return `TRUE` out of the validator.