Description:
------------
When the new __serialize/__unserialize method are made protected or private, a warning is thrown.
Throwing an Error might be better.

Test script:
---------------
class foo
{
    private function __serialize()
    {
        return [123];
    }

    public function __unserialize($a)
    {
    }
}

$f = new foo();

$s = serialize($f);


Expected result:
----------------
"Fatal error: Uncaught Error: Call to private method foo::__serialize()" or similar


Actual result:
--------------
Warning: Invalid callback foo::__serialize, cannot access private method foo::__serialize() in /var/www/html/test.php on line 22