This file is a tool which is used to check SMF source files for common errors, such as files not beginning with the php opening tag <?php and not ending with closing tags ?>. The file also checks for files saved as UTF-8 BOM. For more information on BOM, take a look at How do I get rid of a Byte Order Mark.

This tool can be used to diagnose forums with various issues, such as CAPCTHA verification images not showing. Simply upload the file to the forum's root directory (the same directory as SSI.php), and navigate your browser to:

http://www.example.com/forum/file_check.php

The file can be downloaded here.