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.
http://php-osx.liip.ch/
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.