tidy is a global class, so needs a global namespace specification.

memcache_ui.php

@@ -15,7 +15,7 @@ namespace Memcache_UI {
         'sort-attributes' => 'alpha',
         'wrap'            => 200,
     );
-    $tidy = new tidy();
+    $tidy = new \tidy();
     $tidy->parseString($html, $config, 'utf8');
     $tidy->cleanRepair();
     $html = (string) $tidy;