The bcsub function is a part of the BCMath Binary Calculator and, by default,default Dreamhost PHP installations do not enable the bcmath module. While the libbcmath library is bundled with PHP (since PHP 4.0.4), these functions are only available if PHP was configured with --enable-bcmath, and DreamHost default installations of PHP are no configured that way.
DH does not install custom PHP configurations for customers' use so, if you want to enable the bcmath module, you will need to do it yourself or find someone to do it for you.
DreamHost allows you to install your own version of PHP, and you may certianly enable the bcmath module in such an installation if you choose to do so.
You can learn more about enabling the bcmath module at:
Here are several DH wiki pages that can help you understand how to do this in the DreamHost environment:
If this is not something you feel comfortable attempting yourself, you might find someone else to do it for you; I know Mousee has indicated he does such things.