Hello sbillard. Thanks for replay

You're quite right: if there is index.html and index.php in one directory on "my" server, always the first one is chosen (problem is, that it isn't 'de facto' MY, and so I can't configure it

) But this time I'm sure, that there is only one index.php.
I can't answer your question about value of WEBPATH without modification. I have no technical knowledge about PHP and I don't know how to check it. This was the first thing, I wanted to check, when I found this constant in the source, but I didn't know how to do it. So I started to look for it in the files, and when I found it, i tried to change it.
If there is any method to track PHP script from client side I can change things back and check the value of WEBPATH