Instantiate a new class from a variable's name in a namespace

Let's say for an instance I have a class:

//no namespace
class User { 
    //...
}

And I have a variable:

$model = 'User';

How do I instantiate a new User when I am currently in a namespace?

new $model works when I'm not in a namespace. But what if I am in a namespace and User is not in a namespace.

Something like this doesn't work:

namespace Admin;

class Foo {
    function fighter($model)
    {
        return new \$model;
        // syntax error, unexpected '$model'
    }
}

}