Problem : replacing of variable with it's value in a string(query)

I have my sql-queries saved in a database. Every time I want to run a query I fetch the desired query from the db. It runs fine. The problem is when the query is dynamic. Here is one of my queries. SELECT * FROM DataTypes WHERE type = '$type' I would like the variable to be replaced with its value before the query is executed. How do I do that it. I have tried the query with both the single, double and no quotes. No luck! !! Help me out !! Bob