I have the same, frustrating, problem. Windows, Apache, & PHP 5.3.5 with no luck. I think it has something to do with the build as most advice discusses building PHP in linux. I tried to override with the php.ini but the best I could get was "Fatal error: Call to undefined function ibase_connect() in E:\Apache2.2\htdocs\ibshow.php on line 9".
Just don't know why PHP has to be so complex.