You mean besides DBI?

The different database engines are exactly that: different. They implement basic features, such as DATE and TIME in the SQL standard differently. See this article.

Furthermore, for better concurrency, many database engines require the use of a database specific programming language: PL/SQL (Oracle), SQL PL (IBM DB2), even MySQL has this ability as of version 5.0. While there's some similarities within these languages, they are highly incompatible. So I would say what you are trying to do is very difficult.

Why is database independence so important?