>most installations are fairly "normal,"

To expand on this a bit, if you're a hosting company, you have pretty good incentive to use a set-up of PHP which is widely compatible, because you're not going to want to deal with support tickets from every schmuck who follows the installation directions for phpBB to the letter but still can't get it to work because you've tweaked some esoteric setting or other. Most of those settings are for people who are running their own server and need to specifically tweak PHP just for their needs. This is why, aside from some common tweaks (magic quotes namely... safe mode has no effect on 99% of scripts), having to adapt your script for different PHP environments isn't an issue.