I don't know what I've done wrong.. Looked around in the documentation (a whole day...) but cannot find. I hope someone can help me. This is what I want and have tried:

Files unzipped to: http://www.officeprograms.nl/css
sample html file: http://www.officeprograms.nl/x.htm

Attempt 1: Refering (in the external style sheet css/main2.css) to the PIE.htc file like this:
behavior: url(css/PIE.htc)
No effect.

Attempt 2: Maybe not correct Content-Type...
behavior: url(css/PIE.php)
In the php file I also altered: include( 'css/PIE.htc' )
No effect.

Attempt 3: refering to js file...
behavior: url(css/PIE.js)
No effect.

Attempt 4: I tested the page with IE9 (menu Tools, devellopper tools (F12)). Perhaps this is insufficient, and also tested on my notebook wher IE8 is installed.
No effect.

What I am doing wrong??? What do I overlooked? Please help.

http://www.officeprograms.nl/css/PIE.php is returning a file with zero bytes. I see that PIE.htc is present in the same directory, so perhaps there's something wrong with your server's php processor where it isn't including PIE.htc correctly?

Also, you shouldn't have to modify the contents of PIE.php at all, as long as PIE.htc is in the same directory.

Thanks Jason! The problem was indeed my change in the php file. I thought it was logical. Stupid me...