doesnt replace check to see if there is a duplicate primary key.. if so update, if not insert.. well if the primary key is auto incremented, it would never be duplicate..
I never use replace for this type of thing, but i just thought it could be cleaner if it would work, but maybe im way off.
http://dev.mysql.com/doc/refman/5.1/en/replace.html wrote:REPLACE works exactly like INSERT, except that if an old row in the table has the same value as a new row for a PRIMARY KEY or a UNIQUE index, the old row is deleted before the new row is inserted. See Section 13.2.4, “INSERT Syntax”.