I wanted do some number crunching with the GNU MP Libarary on my Macbook, but it wasn't available by default.
Installation was pretty simple though.
1) Run php --version to check what version of PHP you have.
2) Download that version of PHP somewhere on your system.
3) phpize & compile it
# cd php-5.3.15/ext/gmp # phpize # ./configure # make # make install
4) Add the following to your php.ini