Several options exist for handling web server errors. For example, rather than showing the standard ugly error page that pops up when you reach a website that doesn't currently exist you can make your error page pretty! Useful! Fancy!
The default method, when using the "Error Pages" option in the cPanel Control Panel, are SSI files. This creates files for each type of http error. See the Control Panel for additional details and use.
It may be desired to write these error handlers in a different language, such as PHP, as shown below.
This example will assume a PHP error handler, and assume the root directory of the domain. The filenames of these files can be anything you choose. So within .htaccess place the (or an edited version of) the following:
ErrorDocument 404 /404.php
Now all 404 errors (requests for missing files) will redirect to a file named 404.php and then this file (404.php) can do whatever you wish like show a sitemap, or simply say "Sorry." Each domain or subdomain can have their own.