Windows 2008 R2, Internet Information Services: Changing security settings to change a folder's content by using PHP

Hello,
    
I would like to ask if somebody's there who could help me:
    
I am a PHP developer from Stuttgart, Germany.
    
In my PHP web application I want to edit text files by using PHP code.
These files are created once (by me), so they exist before the PHP application is used by any web user.
My PHP code reads out some text files and other text files' contents are changed.
 
In my developer's environment (Windows XP, XAMPP) it works fine.
So I'm sure my code is OK.
 
But the productive system ist a windows server system (Windows 2008 R2 and Internet Information Services).
And here it doesn't work! The text files' contents aren't changed.
 
I know on windows based webserver systems I have to change the folders security settings
(what I mean: the folders where the text files are placed). I must give the IIS system user (in the past it's name was IUSR..., now it is named otherwise) additional rights, so that it can change folders content.
  
I did. But it doesn't work.
 
Some years ago when we used Windows Server 2003 that was the solution that worked.
Giving the IUSR right to change folders content. That was it.
 
But what is new in Windows Server 2008 that it doesn't work?
  
I think it must be very complicated. Could somebody help me?
 
Thanks
 
Tommy