LIMIT Yourself to MySQL No More

Apache Derby

http://db.apache.org/derby

http://www.amazon.com/exec/obidos/tg/detail/-/0131855255

http://php.net/ibm_db2

SQL Portability

http://sqlzoo.net

http://troels.arvin.dk/db/rdbms/

http://www.oreilly.com/catalog/sqlnut2

http://home.fnal.gov/~dbox/SQL_API_Portability.html

Database abstraction layers

http://adodb.sourceforge.net

http://pear.php.net/mdb2

http://pear.php.net/db

Standard PHP API for database access

http://php.net/pdo