No database access abstraction, so there's no hope for trying to bolt it up to (for example) PostgreSQL instead without heavy modifications. One site (warning, Spanish ahead) mentioned that they were working on creating an abstraction layer, but it doesn't look like they've released anything yet. <RANT>This has got to be PHP's single biggest failing: lack of a project- independant transparent database API, such as Perl's DBI/DBD scheme, or Python's DB- API.</RANT>
Actually, PHP4 is starting to get a very decent DB abstraction layer in the form of PEAR, which is its form of Perl's CPAN.
If you install the PEAR that comes with the latest PHP4, look in /usr/local/lib/pear/DB.php and check it out. The current development version of IMP is using it now, and everything works rather well.