try enable "Copy Firebird client library to <system> directory" option while intalling Firebird
(i tried ok, for wampserver 2.2a and firebird 2.5.0, both 32bit)
seems both "fbclient.dll" and "gds32.dll" is needed in system environment variable
more reference on php.net:
[
hk.php.net]
check out the note by "hiddenmailbox at yandex dot ru" on "15-Dec-2011 12:06"