(Once again a note for myself, but if you find it useful, well, you’re welcome!)
After a PHP upgrade, I noticed at least one bit of PHP code one the server wasn’t working (the TextLinkAds plugin for WordPress) so after a bit of digging around in php.ini, I found that this line was uncommented:
error_reporting = E_ALL
Which was causing warnings to spit out when some PHP scripts were run. I commented that line, and uncommented this line instead…
error_reporting = E_COMPILE_ERROR|E_RECOVERABLE_ERROR|E_ERROR|E_CORE_ERROR
…for a little bit less error reporting/warnings about things, and that fixed it.
I’m sure this is not a cure-all, as another site with the same plugin always worked fine, and was never affected, but still, I didn’t really like seeing those warnings (which I believe are harmless, but they may need investigating anyway.)