Description:
------------
When using spl_autoload_register with two instances of the same class name, the autoload stack only stores one function callback instead of two.


Reproduce code:
---------------
class MyAutoloader {
	function __construct($directory_to_use) {}
	function autoload($class_name) {
		// code to autoload based on directory
	}
}

$autloader1 = new MyAutoloader('dir1');
spl_autoload_register(array($autloader1, 'autoload'));

$autloader2 = new MyAutoloader('dir2');
spl_autoload_register(array($autloader2, 'autoload'));

print_r(spl_autoload_functions());


Expected result:
----------------
Array
(
    [0] => Array
        (
            [0] => MyAutoloader
            [1] => autoload
        )

    [1] => Array
        (
            [0] => MyAutoloader
            [1] => autoload
        )

)


Actual result:
--------------
Array
(
    [0] => Array
        (
            [0] => MyAutoloader
            [1] => autoload
        )

)