restore potentially overwritten variable, fixes #4739

Closes #4739

Merge request studip/studip!3536

lib/plugins/engine/PluginManager.php

@@ -363,6 +363,10 @@ class PluginManager
 
         require_once $pluginfile;
 
+        // Restore variable $class that might have been overwritten by the
+        // previous require
+        $class = func_get_arg(0);
+
         return new ReflectionClass($class);
     }