Add requirement for mbstring extension to be loaded for QR code to work

web/Themes/default/header.inc

@@ -471,10 +471,12 @@ function print_banner(?array $context, $simple=false, $omit_login=false) : void
     echo "</nav>\n";
 
     // Add in a QR code for kiosk mode
-    // (The QR code library requires PHP 7.4 or greater)
+    // (The QR code library requires PHP 7.4 or greater and the mbstring extension)
     if (isset($context['kiosk']) &&
         $kiosk_QR_code &&
-        (version_compare(PHP_VERSION, '7.4') >= 0))
+        (version_compare(PHP_VERSION, '7.4') >= 0) &&
+        //Check for a Mbstring constant rather than using extension_loaded, which is sometimes disabled
+        defined('MB_CASE_UPPER'))
     {
       $url = multisite(url_base() . "/index.php?$query");
       echo '<nav class="qr" title="' . htmlspecialchars($url) . "\">\n";

web/systemdefaults.inc.php

@@ -436,7 +436,7 @@
 $kiosk_default_mode = 'room';
 
 // Whether to show a QR code in kiosk mode
-// Note that PHP 7.4 or greater is required for a QR code
+// Note that PHP 7.4 or greater and the mbstring extension are required for a QR code
 $kiosk_QR_code = true;
 
 // Timeout if the exit kiosk mode dialog is not acted upon