References challenge with PHP4

I've the following code: 1 <?php 2 3 class B { 4 function B(&$a) {

 5     $this->a = &$a;

6 } 7 8 function run() {

 9     print "{$this->a->msg}\n";

10 } 11 } 12 13 class A { 14 function A() {

 15     $this->b = new B(&$this);

16 } 17 18 function &getInstance() {

 19     static $inst;
 20     if(!isset($inst)) $inst = new A();
 21     return $inst;

22 } 23 24 function run() {

 25     $this->b->run();

26 } 27 } 28 29 $a = &A::getInstance(); 30 $a->msg = "Hello world !"; 31 32 // Some tasks, flow control, scope changes, ... 33 34 $a = &A::getInstance(); 35 print "{$a->msg}\n"; // This should print "Hello world !" 36 $a->run(); // This should print "Hello world !" 37 38 ?> My problem is that I don't know what I need to do in order to print the same value in lines 35 and 36. Line 35 prints "Hello world !", but in the 36 nothing appears. What I'm doing wrong ? Thanks for all !