multiple queries in the same "request"

I have a table with some relational values, that really saves me on selects, but gives me a weird error in the insert... in the DB i have this 2 tables data and categories: data( id int category tinyint ), categories ( id tinyint description varchar ) for selects i can use a single query with a left join. but to populate the table without having to use "magic numbers" in my code i've come up with: SELECT (@category_id:=id) FROM categories WHERE description = "cat1"; REPLACE INTO data VALUES( 10, @category_id); i send this as a single query in PHP and it returns an error quoting everything after the first ";" anyidea if i can't send several queries at once? any workaround? Thanks, Gabriel