MDL-67034 lti_provider: php74 fix (curly braces) for LTI Provider lib

lib/ltiprovider/readme_moodle.txt

@@ -13,6 +13,10 @@ These changes can be reverted once the following pull requests have been integra
 * https://github.com/IMSGlobal/LTI-Tool-Provider-Library-PHP/pull/47
 * https://github.com/IMSGlobal/LTI-Tool-Provider-Library-PHP/pull/48
 
+This local changes can be reverted once it's checked that they are present upstream (note the
+LTI-Tool-Provider-Library-PHP repo has been archived so it doesn't accept pull requests anymore):
+* MDL-67034 php74 compliance fixes
+
 It is recommended by upstream to install depdencies via composer - but the composer installation is bundled
 with an autoloader so it's better to do it manually.
 

lib/ltiprovider/src/OAuth/OAuthSignatureMethod.php

@@ -56,7 +56,7 @@ abstract class OAuthSignatureMethod {
         // Avoid a timing leak with a (hopefully) time insensitive compare
         $result = 0;
         for ($i = 0; $i < strlen($signature); $i++) {
-            $result |= ord($built{$i}) ^ ord($signature{$i});
+            $result |= ord($built[$i]) ^ ord($signature[$i]);
         }
 
         return $result == 0;