Invoking a PHP function after all objects have been destroyed

I have seen several answers on object destruction order, and all point out that order is not guaranteed. Since I cannot control the order, I would like to invoke a function after all objects have been destroyed.

register_shutdown_function is invoked prior to object destruction, thus not an option. I have looked at tricks like set_error_handler using the object so it is invoked "late", but that is not sufficient.

Some background on the problem, this is a complex CMS with dozens of separate files for routes (view) layer. There is a common boot include, but not a common one run at shutdown. I am using APCu object caching through a common inherited base class, and need to make sure an object is purged. It is possible that for any two instances of the same object created during a page load, one might want to purge itself, and the other might want to cache itself. Obviously purge trumps all else, so I need to call apc_delete on a global set of cache keys to purge one all __destruct()'ion is complete.

As I said in my comment above, the mixed state of multiple object instances sounds like a design flaw. I think you always want all instances of an object to be the latest, or at least not touch the cache if they aren't. I would think your caching and/or object loading layer could handle this.

To answer your underlying question, I wrote a script to test PHP's shutdown sequence:

<?php  /* Adapted from https://evertpot.com/160/ */

    ob_start(
        function($buffer) {
            return $buffer . "output buffer flushed\n";
        }
    );

    $empty = function() {
        return true;
    };

    $close = function() {
        echo "session close\n";
        return true;
    };

    $write = function() {
        echo "session write\n";
        return true;
    };

    session_set_save_handler($empty, $close, $empty, $write, $empty, $empty);
    session_start();

    register_shutdown_function(
        function() {
            echo "register_shutdown_function\n";
        }
    );

    class MyClass {
        function __destruct() {
           echo "object destructor\n";
        }
    }
    $myObject = new MyClass;

The output:

register_shutdown_function
object destructor
output buffer flushed
session write
session close

It seems the flush of output buffering and session write/close are 2 places where you know all of your objects have been destroyed. (This also assumes you're not using the newer form of session_set_save_handler that can register itself as a shutdown function.)