Convert CamelCase to under_score_case in php __autoload()

PHP manual suggests to autoload classes like

function __autoload($class_name){
 require_once("some_dir/".$class_name.".php");
}

and this approach works fine to load class FooClass saved in the file my_dir/FooClass.php like

class FooClass{
  //some implementation
}

Question

How can I make it possible to use _autoload() function and access FooClass saved in the file my_dir/foo_class.php?