PHP 4.3.11, call_user_func and instances of classes

[David Otton <phpmail@xxxxxxxxxxxxxxxxxxxxxxx>]

A bit of an oddity, this. There's some example code attached which
illustrates my problem.

I am attempting to call a method of an instance of an class from
outside that instance, using call_user_func().

What's happening is that my attempt to call

array ($this, 'AddOne')

is silently being rewritten into a call to

array ('Test', 'Addone')

in other words, instead of calling $test->AddOne I'm calling
Test::Addone. Thus, my expected output:

Add:1,Add:2,Add:3,Subtract:2,Subtract:1,Subtract:0,

becomes

Add:1,Add:1,Add:1,Subtract:-1,Subtract:-1,Subtract:-1,

So, my question is twofold:

a) How can I accomplish this?

b) Why is PHP silently modifying my code to mean something I didn't
write, rather than throwing up an error?

<?php
    $addition = $subtraction = null;

    class Test {
        var $x;
        function Test ()
        {
            global $addition, $subtraction;
            $this->x = 0;
            $addition = array ($this, 'AddOne');
            $subtraction = array ($this, 'SubtractOne');
            doMath('+'); doMath('+'); doMath('+');
            doMath('-'); doMath('-'); doMath('-');
        }
        function AddOne ()
        {
            $this->x++;
            echo ("Add:".$this->x.",");
        }
        function SubtractOne ()
        {
            $this->x--;
            echo ("Subtract:".$this->x.",");
        }
    }
    
    function doMath($choice)
    {
        global $addition, $subtraction;
        switch ($choice)
        {
            case '+':
                call_user_func ($addition);
                break;
            case '-':
                call_user_func ($subtraction);
                break;
        }
    }
    
    $test = new Test();
?>

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