Turns out, as of a few versions ago, #PmWiki has silently introduced a dependency on the intl extension... which is no longer included in #PHP 8.2 and later, and not installed by default on cPanel hosts. For now I can still switch back to 8.1 on the impacted subdomain. In the long run, I'm screwed.
@notimetoplay This seems strange on all fronts. Is there some replacement that 8.2 is deprecating intl? Wait, not included? Is it obsolete? Also strange to add a dependency without making it clear. Good luck.
@JigmeDatse Thanks. No, it's just an extension that used to be installed by default so everyone's come to depend on it, and apparently never heard it's not there anymore.

@notimetoplay It's still included with PHP, with no intention of removing it: https://github.com/php/php-src/tree/master/ext/intl

I don't think it ever installed by default, as it relies on a (common) external library, icu.

Can't say anything about how cPanel handles this though.

php-src/ext/intl at master · php/php-src

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@derickr It handles the way I wrote. Please believe people when they raise an issue.
@notimetoplay My shared hosting account has a section in cPanel to enable extensions. It's important for projects to clearly declare these dependencies.
@kAlvaro Mine doesn't. Imagine that. And like I said: in PHP 8.1 and earlier it's enabled by default.
@notimetoplay Too bad. And I don't have anything super fancy, it's just a cheap plan.