ADOdb is a database abstraction library for PHP and Python. It allows developers to write applications in a fairly consistent way regardless of the underlying database storing the information. The advantage is that the database can be changed without re-writing every call to it in the application. From the ADOdb website, it supports the following databases...
MySQL, PostgreSQL, Interbase, Firebird, Informix, Oracle, MS SQL, Foxpro, Access, ADO, Sybase, FrontBase, DB2, SAP DB, SQLite, Netezza, LDAP, and generic ODBC, ODBTP.

It is important to note that ADOdb uses SQL. Since each database implements SQL slightly differently, the developer will need to be aware of the database-specific features and functions to avoid if they want to maintain portability.

Links

ADOdb home page

phpLens has a comparison of the database abstraction layers, including ADOdb