Which of the following function signatures is correct if you want to have classes automatically load

Which of the following function signatures is correct if you want to have classes automatically
loaded?

A.
functionautoload($class_name)

B.
function __autoload($class_name, $file)

C.
function __autoload($class_name)

D.
function _autoload($class_name)

E.
functionautoload($class_name, $file)