I think it might be because if you write something like $PARAM_INT_sometypo that variable would be undefined, and it is easy to catch. But if you are using constants PARAM_INT_typo becomes a string[1]. That way you would have to do more validation about that, too.

But without more context it's not that easy to tell.

[1]: http://us.php.net/manual/en/language.constants.syntax.php (search for "undefined constant")