bccomp() function belongs to BCMath package; if it's undefined, apparently, there's something wrong with the package itself. To be more specific, it's not enabled (as since PHP 4.0.4, libbcmath is bundled with PHP).

Quoting the docs:

These functions are only available if PHP was configured with --enable-bcmath.

The Windows version of PHP has built-in support for this extension. You do not need to load any additional extensions in order to use these functions.