It's fixed now, but a "quick" update to the PHP engine underneath this blog caused a failure because it suddenly failed at reading the configuration file. Checking the PHP info page showed the path was correct, and the file existed, and the parameter causing the failure was set within it, but the file wasn't loaded.

Adding the following line to the Apache configuration after loading the PHP module seems to have forced it to read the file:

PHPINIDir /usr/local/lib/php.ini

Sadly, reminding myself on this blog will be a little fruitless, as the blog doesn't work when the configuration isn't read...whoops.