⚡ Quick Fix (TL;DR)

The Culprit: mb_convert_encoding() with 'HTML-ENTITIES' as the target encoding was deprecated in PHP 8.2. Any server that upgrades its own PHP version will start logging this notice on unchanged code that previously ran silently.

The Fix: Replace it with mb_encode_numericentity(), which does the same job – converting non-ASCII characters to numeric HTML entities – without triggering the deprecation. Avoid htmlspecialchars() as a substitute here if the content contains real HTML tags you still need parsed afterward, since it would escape those tags too.

$convmap = array(0x80, 0x10FFFF, 0, 0xFFFFFF);
$safe_content = mb_encode_numericentity($content, $convmap, 'UTF-8');