Actually, there is nothing particularly special about .php or .inc
However, when one installs PHP onto a webserver, .php (and .phtml, if I remember correctly) become special in that files on the webspace with such an extension is parsed for PHP code.
That doesnt apply to .inc files by default.
However, people may use this extension merely to differentiate between "normal" PHP files and PHP files to be included.
You could just as easily use .txt, but then there wouldnt be any advantage in differentiation.
However, since .inc files are not parsed for PHP by default, it may be better to place them outside the document root, or use .php files with an "inc" part, like:
filename.inc.php
filename_inc.php
