Not shipped with PHP, but a driver that links with fbclient.dll does
exist:
http://www.fyracle.org/showstory.php?id=350

It still registers itself as "interbase" with PHP, but we're working on
that. We'll probably end up with something like dual "interbase"
and "firebird" registration, using fbclient as the lib for both.

For more details see the discussion on firebird-php.

Paul