I've seen a PHP BC-MATH installation for YUM but not apt-get. Does one know of a working apt-get solution?
Unfortunately, from this SO thread, apt-get install php5-bcmath gives the error Unable to locate package php5-bcmath.
php5-fpm in Precise and later (as well as the Apache equivalent libapache2-mod-php5 package for Apache built-in PHP5 modules) is listed as having bcmath built in to it as an included module. This information is written in the package description file (debian/control) in the package itself when extracted and being viewed as the raw source package, containing source code and a debian/ directory which contains directives to debian build systems to build packages.
Given that the php5-fpm system as well as the libapache2-mod-php5 plugin for Apache has it included by default, I don't think you have to install any additional packages or plugins to get bcmath.
In case my word isn't enough and you want to see the evidence of this, then...
This is a list of links to the debian/rules file which defines which extensions are enabled in PHP at the time of building for each currently supported release at the time of this post (there may be updates after this post that are not reflected here). You can click each of these to see the ./configure arguments that are passed to php5 during the building process for those packages. Note that this will show you specifically where bcmath is enabled for all php5 binaries in those releases:
debian/control file for php5 in Precisedebian/control file for php5 in Quantaldebian/control file for php5 in Saucydebian/control file for php5 in Trusty
