It does not support multiple databases with a single application (although you can, with caution). This does not make it easier to change databases in the future (although this helps). It is that one consistent, reasonable interface is used regardless of the database. This not only helps programmers (more widely applying their skills) and projects (simplifying the transition of programmers), but also greatly simplifies the creation of libraries that are located at one level above the level of data access. Perl has had DBI for 20 years, and that was very good. PDO is a very similar concept (in fact, it steals at least half of the DBI).
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