On the Debuggable blog, Felix Geisendorfer has a quick post showing the (basically) one step installation of the latest development version in the PHP 5 series, PHP 5.3.

I'll share an easy way to play around with PHP5.3 if you're on a Mac. First of all install MacPorts (don't forget to install XCode before).

He also includes a one-liner to be sure its all installed and working with the right version.