Description:
------------
When trying to call the register_shutdown_function() inside class like this:

register_shutdown_function(array(&$this, "_log"));

and the method _log() is a private method it don't work. We assign $this here to the function so 
register_shutdown_function() should be allowed to call a private function inside of the class or at least throw allready here an error message when this function is called and _log() is a private function. Its also not documented that this wouldn't work....

Reproduce code:
---------------
<?php
final class TestClass
{
        public function TestClass()
        {
                echo "Class init call!\n";
                $this->_init();
        }

        private function _init()
        {
                echo "Registrering shutdown function..\n";

                //-- register log function
                register_shutdown_function(array(&$this, "_log"));

                echo "done\n";
        }

        private function _log()
        {
                        //-- send mail
                mail("thomas@ecommerce.com", "subject", "content");
        }
}

echo "Script start! Init Class.\n";
$TestClass = new TestClass();
echo "Class inited!\n";

Expected result:
----------------
email will be send to thomas@ecommerce.com when script is done.

Actual result:
--------------
Script will not call the shutdown function