After much trial and error i found out you should use the following to connect to a firebird database:
<?php
$db = new PDO ("firebird:dbname=localhost:C:\\path\\to\\database\\MyDatabase.FDB", "username", "password");
?>
NOTE: in the 1st part of the connection string there can be NO spaces else you will get NULL.
The string provided on this site does not work and will give no error only NULL.