From a4ea607c24d22d9ca8aa129e3052a02adc427efe Mon Sep 17 00:00:00 2001
From: Andrew Nicols <andrew@nicols.co.uk>
Date: Fri, 6 Jan 2023 22:37:07 +0800
Subject: [PATCH] MDL-76362 core: Short circuit s() on empty values

---
 lib/weblib.php | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/lib/weblib.php b/lib/weblib.php
index 10b27d82cda..45b00ef4e69 100644
--- a/lib/weblib.php
+++ b/lib/weblib.php
@@ -91,13 +91,18 @@ define('URL_MATCH_EXACT', 2);
  * @return string
  */
 function s($var) {
-
     if ($var === false) {
         return '0';
     }
 
-    return preg_replace('/&amp;#(\d+|x[0-9a-f]+);/i', '&#$1;',
-            htmlspecialchars($var ?? '', ENT_QUOTES | ENT_HTML401 | ENT_SUBSTITUTE));
+    if ($var === null || $var === '') {
+        return '';
+    }
+
+    return preg_replace(
+        '/&amp;#(\d+|x[0-9a-f]+);/i', '&#$1;',
+        htmlspecialchars($var, ENT_QUOTES | ENT_HTML401 | ENT_SUBSTITUTE)
+    );
 }
 
 /**