php - PHP: 参照によるクラス メンバーの受け渡しが機能しない

Question

この質問で、変数の変更を登録するイベント ハンドラーを求める人がいます: PHP 変数の変更を検出する方法は?

PHPの魔法の関数__getと__set. これは、参照によってメンバーを通常の関数に渡すまで機能し、イベントをトリガーしなくなります。

これはバグですか、それとも不可能なことですか?

<?php
header("content-type: text/plain");

class WatchVar {
    private $data = array();
    private $org = array();
    private $callbacks = array();

    public function __set($name, $value) {
        if (!array_key_exists($name, $this->data)) {
            $this->org[$name] = $value;
        } else {
            //variable gets changed again!
            $this->triggerChangedEvent($name, $value);
        }
        $this->data[$name] = $value;
    }

    public function &__get($name) {
        if (array_key_exists($name, $this->data)) {
            if ($this->data[$name] != $this->org[$name]) {
                //variable has changed, return original
                //return $this->org[$name];
                //or return new state:
                return $this->data[$name];
            } else {
                //variable has not changed
                return $this->data[$name];
            }
        }
    }

    public function addCallback($name, $lambdaFunc) {
        $this->callbacks[$name] = $lambdaFunc;
    }

    protected function triggerChangedEvent($name, $value) {
        //$this->data[$name] has been changed!
        //callback call like:
        call_user_func($this->callbacks[$name], $value);
    }
}


$test = new WatchVar;
$test->addCallback('xxx', function($newValue) { echo "xxx has changed to {$newValue}\n"; });
$test->xxx = "aaa";

echo $test->xxx . "\n";
//output: aaa

$test->xxx = "bbb";
//output: xxx has changed to bbb

echo $test->xxx . "\n";
//output bbb

function messyFunction(&$var) {
    $var = "test";
}

messyFunction($test->xxx);
//output: nothing, why?