из мануала:
If you get a message like: \'Dynamic SQL Error SQL error code = -901 feature is not supported\', this may be caused by InterBASE client 6.01 accessing a server running 5.x.
Make sure the dialect value is set to \'1\', like the following:
ibase_connect (\'localhost:/usr/db/db.gdb\', \'SYSDBA\', \'masterkey\', \'ISO8859_1\', \'100\', \'1\' ) ;
That last value is the dialect, but you need the two preceding also, do avoid errors.
Anthony