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.
