I feel this is an awfully obtuse question to ask, but strangely, this problem is poorly documented.
I would like to upgrade PHP, but there are several problems:
Option #1
As recommended here, this site provides a convenient, up-to-date one liner.
This doesn't overwrite the base version of PHP on your system, but instead installs it cleanly in /usr/local/php5.
Option #2
My preferred method is to just install via Homebrew.