Prevent inheritance corner-cases

What if a PHP 5 project defines Error? And it's a singleton, not an exception? And we need to throw a TypeError?

This resolves that corner case.

lib/error_polyfill.php

@@ -35,8 +35,15 @@ class Error extends Exception
 }
 
 if (!class_exists('TypeError', false)) {
-    class TypeError extends Error
-    {
-
+    if (is_subclass_of('Error', 'Exception')) {
+        class TypeError extends Error
+        {
+
+        }
+    } else {
+        class TypeError extends Exception
+        {
+
+        }
     }
 }