Type really shouldn't make a difference here, since, when it's passed to the db, it's all a string anyway. But to make sure, you can always just drop $year and add 1992 in the code and see what the return is. I'm betting it will be the same result, in which case you will have to look elsewhere for the flaw.
-Syn0

update: damn, rchiav beat me :) .. another thing, when forcing the type, you dont' want to do it before you declare what's in $year, you want to do it during or after.
for example: $year = 1992; $year = int($year); or $year = 1992+0;