通过字符串classname调用类

I seem to be stuck on a very basic PHP issue.

        $test = new TestClass;

Works and creates an object of TestClass.

        $class = 'TestClass';
        $test = new $class;

Fails with the error Uncaught Error: Class 'TestClass' not found.

Why is this failing? I used this a lot in the passed and then it always worked. Could it be because of the Namespaces? How do I need to load it then?

All these classes are in the same namespace and are being called from the same namespace