Introduced check before sorting to ensure core_collator class exists as it was only introduced in 2.6

lib.php

@@ -403,7 +403,7 @@ function customcert_get_elements() {
         }
     }
 
-    core_collator::asort($options);
+    customcert_perform_asort($options);
     return $options;
 }
 
@@ -969,3 +969,16 @@ function customcert_generate_report_file($customcert, $users, $type) {
     // Close the workbook.
     $workbook->close();
 }
+
+/**
+ * Perform asort on a given array.
+ *
+ * @param array $fields
+ */
+function customcert_perform_asort(&$fields) {
+    if (class_exists('core_collator')) {
+        core_collator::asort($fields);
+    } else {
+        collatorlib::asort($fields);
+    }
+}