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:
locate php
indicates there are probably many dependencies.I am running on Leopard. I have a feeling Apple doesn't want you to upgrade. Would buying Snow Leopard and upgrade solve this problem (and future ones like it)?
You may want to check out Marc Liyanage's PHP package. It comes in a nice Mac OS X installer package that you can double-click. He keeps it pretty up to date.
Also, although upgrading to Snow Leopard won't help you do PHP updates in the future, it will probably give you a newer version of PHP. I'm running OS X 10.6.2 and it has PHP 5.3.0.