Originally Posted by wige

This is done server side, you need to set the mime type of the file through .htaccess if it is an Apache server. However, I would not be suprised if the wiki system used mod_rewrite to change /some_entry to /showEntry.php?s=some_entry. Because Apache knows what mime type to send for the PHP file, no modification has to be made, even though the outside user doesn't see the extension.