It should be __autoload()
seems like you missed an underscore character.
That is..
function __autoload($class_name) {
require_once $class_name . '.php';
}
$a = new Cont();
A tip from the PHP Manual...
spl_autoload_register() provides a more flexible alternative for autoloading classes. For this reason, using __autoload() is discouraged and may be deprecated or removed in the future.