Add final keyword to both classes

MD_STD.php

@@ -8,7 +8,7 @@ declare(strict_types = 1);
  * Standard class providing overrides of default PHP functions as static
  * functions.
  */
-class MD_STD {
+final class MD_STD {
 
     /**
      * Wrapper around file_get_contents, that provides catches errors on it and returns