check if JsonException exists before adding it to the global namespace

Author: digibeuk

compat/json-exception-polyfill.php

@@ -1,6 +1,6 @@
 <?php
 
-if (PHP_VERSION_ID < 70300) {
+if (PHP_VERSION_ID < 70300 && ! class_exists('JsonException')) {
     class JsonException extends Exception
     {
     }