Know if the class can be instantiated

I have a concatenation like

list($dummy, $class, $action) = split('/', $_SERVER['PATH_INFO']);
$class = ucfirst($class) . 'Controller';
$controller = new $class;

So if I'll pass /post I can have PostController but if the PostController doesn't exist, how do I know that so that I can't new $class coz it gives Cannot instantiate non-existent class: postcontroller?

Thanks in advance.

Code:

---

if (class_exists($class))
  $controller = new $class;
else
  // ...

---

Thanks PG