The BC functions allow basic mathematical operations to be performed on very large numbers that would normally not be possible on standard computer architectures. With standard mathematical operations, computers can only process numbers of a certain size; when that maximum size is reached, the number wraps around to its lowest value. Of course, this can lead to mathematical errors in your code. Using BC functions fixes this problem. One issue with these functions, however, is that they're not available by default in PHP due to licensing restrictions, and must be downloaded separately from http://www.php.net/extra/number4.tar.gz. As of PHP 4.0.4, license conditions for the library changed and it is now available as part of the PHP source.
PHP Functions Essential Reference. Copyright © 2002 by New Riders Publishing (Authors: Zak Greant, Graeme Merrall, Torben Wilson, Brett Michlitsch). This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/). The authors of this book have elected not to choose any options under the OPL. This online book was obtained from http://www.fooassociates.com/phpfer/ and is designed to provide information about the PHP programming language, focusing on PHP version 4.0.4 for the most part. The information is provided on an as-is basis, and no warranty or fitness is implied. All persons and entities shall have neither liability nor responsibility to any person or entity with respect to any loss or damage arising from the information contained in this book.