> The files are all PHP-Files like index.php?id=173, but 
> httrack generates mysterious filenames like 
indexfddb.html.

The appeneded characters are generated according to the 
query string checksum (this is the 16 first bytes of md5
(querystring), without the "?")

> Which rules does httrack use to generate the filenames?
You can change this behaviour using "Build structure" 
options.