How can I call a static method from a class if all I have is a string of the class name?

Question

How would I get something like this to work?

$class_name = \'ClassPeer\';
$class_name::doSomething();

Answer 1

After I have almost missed the simplest solution from VolkerK, I have decided to extend and put it in a post. This is how to call the static members on the instance class

// calling class static method
$className = get_class($this);
$result = $className::caluclate($arg1, $arg2);

// using class static member
foreach ($className::$fields as $field) {
  :
}