fix(UserConfig): add comment explaining the (string) cast and PHP OPcache bug

Reference php/php-src#21973 so the cast is not removed by static analysis
or cleanup automation until the minimum PHP version is bumped past the fix.

Author: thereisnotime

lib/private/Config/UserConfig.php

@@ -705,6 +705,10 @@ public function getValueBool(
 		bool $default = false,
 		bool $lazy = false,
 	): bool {
+		// The explicit (string) cast guards against a PHP OPcache bug where values passed
+		// by reference across function boundaries can have their type corrupted (e.g. bool
+		// returned as int). Affects PHP 8.x with OPcache enabled; fixed upstream in
+		// https://github.com/php/php-src/pull/21973. Keep until minimum PHP version is bumped.
 		$b = strtolower((string)$this->getTypedValue($userId, $app, $key, $default ? 'true' : 'false', $lazy, ValueType::BOOL));
 		return in_array($b, ['1', 'true', 'yes', 'on']);
 	}