Defining constants with a value of 0?

I'm running into some unexpected behavior that I can't figure out. This is with PHP version 3, where, to my knowledge, boolean is not built in. <?php define ("true", 1); define ("false", 0); echo gettype(true); echo gettype(false); ?> The type of true is returned as an integer, while the type of false is returned as a string. Even if I say (int) 0, it still returns its type as a string. Can anyone explain to me why it behaves this way? Thank you. -Rob