Please don't misunderstand me here I do appreciate your suggestion as all ideas are some merit which may suit or may not.
But please check this out first.
I am no expert in php but if you look at this link http://www.ozyautos.com.au/loan-calculator.php
which is a as you would already know a direct link to the loan calculator and try it there it will work exactly as it should.
So if it's working on my server out of the same root dir but not when it's been called with my script wouldn't you say that the most likely reason it's not working would be because the 2 scripts are conflicting somewhere and it's just a matter of finding out where as Aerospace was able to do above.
I have looked at calculator scripts for over 2 weeks now and this is by far the best one I have seen and it actually works where many others don't even on heir own demo sites.