If you’re trying to install the Open ID PHP library the first thing you hit is:
1. Install dependencies. - Enable either the GMP extension or Bcmath extension. (GMP is STRONGLY recommended because it's MUCH faster!) This is required.
All my attempts to find how to install GMP on Ubuntu led me to long pages of having to compile libraries and re-compile PHP. Whilst they were doing their bit at the time it really is much easier now. Ubuntu has the php5-gmp library included, so you can just use apt-get.
$ sudo apt-get install php5-gmp
Bosh. Happy days. Thanks to the Joys of Programming for showing me the way.