<?php
interface Morphable { 
    public function morph($class);
}   
class Morpher implements Morphable {
    private static $pointers = array();

    public static function instance($class = 'Morpher', &$var) {
        $var = new $class();
        self::$pointers[] =& $var;
    }
    public function morph($class) {
        if( ! class_exists($class))
            throw new Exception("Couldn't morph. Class $class doesn't exist.");

        $index = array_search($this, self::$pointers);
        self::$pointers[$index] = new $class;
    }
}
class A extends Morpher { }
class B extends Morpher { }
Morpher::instance('A', $m);
var_dump( $m );
$m->morph('B');
var_dump( $m );

show_source(__FILE__);
?> 

object(A)#1 (0) { } object(B)#2 (0) { }