Code not working in PHP 4

I have a PHP script which works fine in PHP 5, but not in PHP 4. I've made a small test case for you to demonstrate (disclaimer: I know that the below code could be written much better, but it's not an actually used piece, rather the one to demonstrate what I'm talking about):

class Messenger {
    var $messages = '';

    function add($message) {
        $this->messages .= "$message\n";
  开发者_运维问答  }
}

function add($m) {
    if (! isset($GLOBALS['instance'])) $GLOBALS['instance'] = new Messenger();
    call_user_func_array(array($GLOBALS['instance'], 'add'), array($m));
}

add("One");
add("Two");
add("Three");

var_dump($GLOBALS['instance']->messages);

Under PHP 5 the messages property contains all 3 messages, under PHP 4 it is empty. Why?

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In PHP 4, $this does not seems to be work the same way as PHP 5 does.

The $this pseudo-variable is not usually defined if the method in which it is hosted is called statically. This is not, however, a strict rule: $this is defined if a method is called statically from within another object. In this case, the value of $this is that of the calling object. This is illustrated in the following example:

example : http://www.php.net/manual/en/keyword.class.php