Upgrading PHP Difficulties

I'm trying to update the PHP on this server, but it's kicking me in the head a little bit. The newest (and just some newer) versions (5.3.x instead of 5.2.x) compile, build, and seem to install just fine, but the upgraded versions fail to run the PHP software (like this blog), throwing numerous errors in the existing PHP scripts.

I'm still crunching, but in case you noticed a failure trying to visit a PHP-backed site on this server...well, now you know why it failed.