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I just updated to wordpress 6.6 and I got the above error. Went away when I disabled WP Super Cache
This seems to be related to https://core.trac.projectdmc.org/ticket/61182
Full error:
[Wed Jul 17 10:48:12.068500 2024] [php:error] [pid 3188270] [client 115.188.1.183:38506] PHP Fatal error: Uncaught Error: Call to undefined function is_utf8_charset() in html/wp-includes/compat.php:179\nStack trace:\n#0 html/wp-includes/compat.php(152): _mb_strlen()\n#1 html/wp-content/plugins/wp-super-cache/wp-cache-phase2.php(268): mb_strlen()\n#2 html/wp-content/plugins/wp-super-cache/wp-cache-phase1.php(194): wp_cache_serve_cache_file()\n#3 html/wp-content/advanced-cache.php(22): include_once(‘…’)\n#4 html/wp-settings.php(97): include(‘…’)\n#5 html/wp-config.php(77): require_once(‘…’)\n#6 html/wp-load.php(50): require_once(‘…’)\n#7 html/wp-blog-header.php(13): require_once(‘…’)\n#8 html/index.php(17): require(‘…’)\n#9 {main}\n thrown in html/wp-includes/compat.php on line 179